Mobile devices most definitely are “instant on”. Try it. Press the power button, the device is immediately off, press it again, the device is immediately on. Nearly everyone would call that “instant-on”.
That the devices don’t really ever power down completely is merely an implementation detail, not really relevant for the scope of this discussion. (At least no warranting a statement like “most definitely not instant on”.)
Agreed with OP. What makes the Mac Book Air so impressive, is that it's both. An aggressive sleep feature means that, unlike my MacBook Pro, which had an identical work pattern - I have _yet_ to have my MBA run out of batter. The MBP would routinely run out every three-four days.
At the same time, when it _does_ run out of battery - it powers up in 15 seconds. Not _quite_ instant on, but it makes a measurable difference in how you feel about powering it up to use. Ironically - you never do because it's always on. :-)
I would argue that they are, yes. At least to some extent. (I use my laptop that way. My mom doesn’t.)
What has changed is the UI. People are now often encouraged to send their devices to sleep, that’s not really the case with even current laptops. Truly powering down is an equal citizen on desktop operating system instead of some weird function only nerds would use (like it is on smartphones).
I’m not familiar with Android phones. I have never seen or heard of anyone intentionally rebooting or shutting down his or her iPhone or iPod touch, though.
Battery power is also a factor that comes into play here. These devices are "always on" for days at a time. Powering down isn't really the issue so much as leaving your perception. You can forget about them ("turn them off") and when you start using them a day or two later they come on in an instant.
When it comes to a laptop you can't necessarily leave it and come back, you have to be cognizant of how long it's been standing by or hibernating or whatnot.
Right, but the idea is that things are becoming instantly available again. It's not that these devices don't take any time at all from a cold start, but the fact that we're leaving things in standby mode these days. I don't power down my iPhone each time I'm done using it, I just lock it.
Try it. Power down your phone, start the clock and measure how long it takes to boot....
....see?