I'm equal to any "power" user, I work on the road and tether daily while using the iPhone for basically everything. I'm still not getting anywhere near 6GB. You'd literally have to use the iPhone connection as your main internet source to get above 6GB. I'm not torrenting or streaming a TON while on my phone, but I watch mobile streamed hockey (they are at 400 kbps) fairly often.
HD video is indeed more then 300kbs - but are my Youtube videos on the iPhone 1080p? Hell no. Even netflix for mobile is severely compressed and still looks OK.
You may use more then 6GB of mobile data a month, but as I said before, I'm willing to bet you're in an extremely small percentage there.