Well, not all 50gb sites need a dedicated server.
But remmeber, a Lot do. Bandwidth is not the large problem when hosting large sites on a shared server. If a popular 50gb a month site stays on a shared server and runs a popular bbs or has a lot of cgi it will bog the shared server down a lot.
So the main reason for getting a dedicated is not necessarily bandwidth, although it does play a major part, is the fact of CPU usage. Its a fact that sites are becomming more and more CPU intensive than 4 years ago when I started in this industry.
Hope that helped answer your question a bit.