not neccesarily, it's quite possible to make good fake 3d in photoshop. I know, but for the most part a 3d program is better. You can get better abstract, and more variety.
Well, with photoshop, you can do space sigs like you see people around here with. It can do stars, planets, planet rings, nebulae. If you want to be able to do 3d and good abstract, it helps to have something like Bryce.