That is one gripe I don't get. I'd like better cameras as it's always better to have better quality.
But my sole use of them is for video calls and they're apparently fine for that. can't be much worse than the crappy webcame on my thinkpad that I use now.
I have a digital camera that fits in my pocket that I use for pics. I won't carry my iPad around when site seeing etc. anyway as I usually don't have a bag, and it would be awkward standing somewhere and taking a picture with a 10" tablet anyway!
Rear cameras are great for showing people things, whilst being able to seeing their reactions in real-time. It also lets you see what they are seeing through your lense, so you can aim properly. There are also augmented reality aps for iPad (and Android, and Palm devices) which make use of the rear camera. Some video games use a videofeed from the rear camera to move the game view like Arcade Reality, one of the first augmented reality Aps that was popular on the Palm platform.
It would also be handy for capturing paperwork like a photocopier. I use my phone for that quite often, to keep records of paystubs, and important things I have to remember later. Try doing that with a webcam sometime. It sucks!