To the degree a flat design idiom such as Metro [or whatever it's called] begins to admit text and favor it over icons, a reasoned argument can be made that they are objectively better.
It is not just that the pictographic language of icons -, folders, floppy disks, file cabinets, trashcans, glue bottles, scissors, clipboards etc. - is drawn from a context largely irrelevant to large segments computer use today such mobile devices, educational contexts and home uses. Nor is it that elements such as the floppy disk are twenty years obsolescent.
The problem with the pictographic language rooted in skeuomorphism is that it lacks a grammar. It has no rules of combination. There is no icon for copy and paste - let alone one for copy from context a and paste in context b.
It is not entirely an accident that alphabets have been replacing pictograms for three millennia. They are more flexible. Likewise, the iconography of industrial buttons is rapidly disappearing both as a relevant context from the workplace, and as a human interface. Industrial interfaces are increasingly a screen and a keyboard [a keyboard is our industrial interface to an alphabet].
What is the most useful feature of the iOS interface? It would be hard to argue for anything other than the touch keyboard. It has two important features. Each icon represents a symbol rather than an action. Those symbols have rules of combination which allow the expression of complex abstractions.
Your roommate can text "Scrunchie on door." They don't need to download "The Let Someone Know You're Getting Laid App" from the app store. Neither do you.
The icon and skeuomorphism will continue to lose ground simply because they are too brittle for general purpose computing. There will be specialized contexts where they persist - just as pictograms are a useful way to describe the way the Grants Mill Road winds over the hill. The sign before two successive turns shows two successive turns.
Today, we are in a future where Grants Mill Road has been straitened as it heads down toward the bridge, but some interface designer has not only retained the sign but had it painted to simulate a brushed aluminum finish and shading to simulate an embossed surface illuminated from the upper right.
Most skeuomorphism is cutting two inches off the potroast.
This is a very perceptive observation, but you're ignoring the reciprocal trend in the evolution of language: The development of abbreviation, shorthand, and idiom. The conceptual open-endedness of language allows for infinite flexibility, but at a cost of efficiency of communication.
You actually allude to this in your example: "Scrunchie on door" is a cultural idiom for "please do not enter the room as its occupants are currently engaged in intimate activities" – we can imagine the occupant chose an iconographic symbol for this rather than writing out the aforementioned sentence in marker on a piece of paper solely because it was more efficient (there are plenty of euphemisms that could be written out longhand as well, negating any argument that the icon would be chosen purely out of discretion).
Ultimately, this sequence of abstraction be seen as a loop: Ancient pictograms led to written language, which may be optimized to idiom and icon (the scrunchie), which again serve to inform language (texting about the scrunchie). Iconography is not a one-way path to language – it's a communication tool that works in conjunction with language.
The roommate in fact wrote out their message and sent it as text. There need not be an actual door and nobody needs to own a scrunchie. In fact, the message "scrunchie on door" may communicate "whoohoo!" and not "Do not disturb" - i.e. the receiver might be out of the country or the sender at a hotel with a lover. It could even mean, "I found your scrunchie here, and have placed it on the door." Sometimes a cigar is just a cigar.
That's the expressiveness of a general purpose tool. The days when icons were needed to sell computers to men because typing was women's work, are gone. Our human interfaces need to catch up.
I agree with mortenjorck, but putting that aside, I think you're missing the prevalence of pictograms in textual conversation. ;) People keep coming up with more and more stupid faces composed of obscure Unicode characters ( ͡° ͜ʖ ͡°) and there are even some built into the character set itself 😆, along with all kinds of other symbols that I can use to tell a story or communicate an idea in a minuscule amount of space.
Text is a great general-purpose medium for communication (amongst people that speak the same language), but a well designed pictogram:
• Takes up less space than a literal textual translation, sometimes shockingly so ("a picture is worth a thousand words")
• Can transcend cultures much more easily (smiley faces seem to be fairly universal, and someone who had never seen a peace sign before could have the symbol explained to them much quicker than it would take to teach them enough of the English language to recognize the written word "peace")
• Can be more legible at a distance or by those with poor vision
On the other hand, not all ideas can be expressed visually. For these situations, we can also arbitrarily assign meaning to an image, and so it becomes a symbol. Consider that every letter in our alphabet is a symbol: Why does 'a' mean a? Simply because we agree that it does. Why does this make you think, "STOP"?
There's nothing about that symbol that at all conveys the act of stopping, and yet its meaning is ingrained within all of us. For that reason, I think this hubbub about our save icons being outdated is pointless designer hand-wringing. The save icon may have started out as a pictogram representing a floppy disk, but with the death of the floppy disk as a storage medium it has transcended that to become a symbol: an arbitrary image that we agreed at some point to mean "save." Ask a child today that grew up with computers but is too young to remember floppies, and I believe most will tell you it is just "the save button" and not even consider that it is supposed to represent an actual physical object.
There is a need for both words and images in communication, and the intent of interface design is to communicate the means of interacting with the interface to a person, so I believe ideal interfaces would not needlessly be restricted to one or the either. I don't believe that every action deserves an associated symbol (the "Microsoft Office school of design," if you will), but I believe that symbols are useful for conveying meaning, and we should not throw out the ones that people happened to get used to over the years just for its own sake. Plus and minus signs are very useful things to have in the common visual vernacular (for representing adding and removing things from various lists), so eliminating the floppy disc icon seems a bit to me like wanting to eliminate the plus sign.
I'm not sure I'm satisfied with this answer, but I think it indicates where I am headed.
At one end of human interface design, GNU Emacs. It is so flat as to allow [nearly] complete freedom of expression by the user. The price? The ability to deploy an ad hoc set of abstractions creates the possibility of multiple learning curves and requires a sophisticated mental model to be maintained by the user.
At the other end, maybe something like the sliding rheostat on my toaster. It controls one thing directly and proportionally. It persists across machine states [i.e. whenever I am using the toaster I have direct access] and is functional - a given setting produces the same output (in terms of energy, if not of toastiness.)
What isn't good flat design is Apple's new calculator - or rather it isn't good redesign despite being executed in a flat idiom. It removes functionality by eliminating mc m+ m- mr. The end result is that for aesthetic effect, Apple has made the calculator less useful. It has reduced the user's ability to express ideas.
They have used flat design as a cop out. I am sure that they have data which indicates that most iOS'ers don't even know what mr does, let alone uses it. But the real design question is, what would people use, and the answer is probably something like parenthesis and square root and maybe a doubling function and a halving function and something which multiplies by ten...and notice I've got five buttons that they need to stick in their four column design?
See, this is the problem with skeuomorphism. Apple still wants to make something that looks like a pocket calculator, instead of crating an interface to a general computing device.
What would a really useful phone calculator look like?
I've been thinking that the real future is between, User Interface design and Human Interface design. Is the future of our interfaces going to allow expression or only interaction within a loop dictated by the machine? Keyboard Cat can be trained to flip switches, but cannot express a musical composition. I look at the Apple calculator and wonder, where is the Integral key?
iPhone is better at displaying text since it has a taller screen. A virtual keyboard has pagination expand the set of characters available and can also adapt to show certain characters in certain situations, such as a calculator.