I agree with you 100% on everything you said here, so thankyou for helping to show that you're not the only one who feels this way. Video content can be especially large, so Youtube videos tend to guzzle alot of my data when out and about (something very useful for people who travel long distances either to work or on vacation, etc.) I've even known email backups to instantly kill all my data in the background (even with filtering and blocking as much spam email as possible), so I've had to resort to disabling backups in order to not to kill all my data before I've even got to do anything I wanted for myself on my phone. (admittedly it is good practice to limit those types of things, but it's more the having to actually limit useful things that we are most concerned about.
Don't get me wrong, I hear what people are saying about discipline, but I still find that even with the most disciplined of practices, that I still feel very overly limited in the amount of things I am able to do on the go compared to when I'm on a good stable home WiFi connection.
I agree also with all the posters who mentioned that that public WiFi is not always up to par, so can leave us completely at the mercy of how good any given public connection is for wherever you happen to be. (anyone who has ever stayed in Hotels for example in any country, will be well aware of how much this can vary in terms of signal strength). Some are great, and others just outright suck, and you can't always tell that before you book unless you happen to always use that exact same hotel, but can leave you very stuck without internet for anything that requires decent speeds or sizable data. So the comments people have made about over-restriction are definitely valid.
Don't get me wrong, I'm sure this will get better over time. It is definitely a good sign that alot of the USA based iPhone users on here are saying that their limit never runs out due to public WiFi being alot more readily available than in the rest of the world. But even in cities and towns where this is easier to do, wouldn't it still be much more convenient not to have to resort to switching public WiFi connection each time? I'm sure I can't be alone in thinking that the ideal setup would be to just be able to use our iPhone for anything internet that we wanted or needed as freely as we do our own home WiFi connections, wherever we happen to be at the time. This to me would be true mobile internet. So I do hope that we get to see this in the near future.