Your primary question was "What is the usage for a 30-378." You can most certianly hunt with it. I recommend going over to longrangehunting.com and reading some of the boards on this round, those boys live for longe range. On the other hand, i was checking into having one built for myself last year by a smith. It was going to be for long range target primarely. If you check out Arizona Ammunition, you can get a special barnes 150gr bullet, that is shaved down just a hair, they have gotten it up to 3945fps. Thats FAST. So to answer your question, Hunting and Target Shooting are its uses. Better reload though, because it could drain your checkbook otherwise to be able to afford to shoot it.
Nick