I had to show this setting to my fiancée who just got her first iPhone, a 5c, as a free upgrade when her feature phone kicked it, and since she isn't paying for a data plan...
First we figured out how to disable cellular data, since the phone with the minimum data plan gets only 100MB of usage per month, it's definitely too much to ask that one should police their applications and "just not use data" -- I don't have to tell you how easy it would be to accidentally go over this cap if you are actually using apps or any features of the iPhone at all, and the incidental metered usage rates are just absolutely ridiculous.
Then we had to go and find the Send as SMS setting, which was disabled by default, because text messaging to other iPhones only ever worked when she was in a WiFi zone. She would receive a flurry of messages she was missing while out, immediately when she got home, obviously a symptom of this issue with iMessage.
Even now, with the setting enabled, other iPhones won't reach her if they try to send a message when she's not in WiFi zones, because they still have this setting disabled. You have to show all your friends with iPhones how to enable this setting, or to force one message through individually, or just bite the bullet and pay for a real data plan. It does seem to be all by design, some type of collusion with the phone carrier(s) to make more money on data plans.
I agree that iOS and apps could do more to help users stay within the limits of their data plan. Sometimes it seems like they assume data is free.
I once had Spotify eat my entire data for the month in a few hours because wifi was spotty and it just went on to stream (in max quality) over 3G.
Another issue, especially in Europe, is international roaming. When you're near a border, your device may be roaming in the neighboring countries network because their signal is stronger, so you really have to catch the "welcome to foo network" sms and turn off data roaming (or pay for an "EU Day Pack").
This makes sense, I see now that you can disable it. But was this even an option before Apple published the "selfsolve" page referenced here in today's news?
I haven't seen any articles that suggest you should disable iMessage unless you are dropping your iPhone completely. It was totally not obvious to me that you can even do this, and I am professional. ^_^ (I don't have my own iPhone, so I've never had to wrangle with these issues on my own.)
I don't think this is true. I have a phone here I only use for development where I don't think I changed that setting and it's on.