You must have some pretty big pockets!
I find the blackberrys to be really solid units, and I personally do not think a case is necessary if you're putting it in your pocket. The only time I find a case to be really necessary is if the device has a touch screen. They're really sensitive, and you don't want to break it. The blackberry seems like it wouldn't break all that easily if no case is used.
Just my 2 cents