also are people using it within safari for youtube, as well as the youtube app (for HD playback)? i dont have it but the safari youtube looks nearly HD dont see why people would need this aside from facetime
another use is the ability to download/update apps from the app store that are over 20mb in size. The other ones you mentioned are big as well (youtube HD playback, and now facetime is supported).
For me, I like it because ios won't let you download files over 10mb (20mb now I think) and I'm always on the go so it let's me download exceptionally large emails and dl movies from iTunes as well. I used to use it for spoofing Skype to thinking I was on wifi but I have no clue if that's even necessary as I haven't used Skype in ages.
My power went out a few months ago for 4 days and obviously I had no wifi. I enabled 3gunrestrictor on itunes, plugged in my phone to my car for a few hours and downloaded a few movies. This enabled me to watch some stuff while the power was off and listen to some podcasts.