I don't allow/run psionics for the following reasons:
1) They're pretty pointless in 3.0e unless you make psionics a part of the campaign. So many powers either attack or defend against other psionics that I feel I have to throw in psionics more often than I would want. While this is true to a lesser degree with Rangers (if you never throw in their favored enemy they're eventually going to feel cheated), with Psionics 3.0 you have to bring in a whole new set of rules just for one or two PCs.
2) While I've heard that 3.5e fixed a great deal of problems with psionics, it still means I have to buy another fairly expensive book just on the off chance someone wants to play a psionic. I decided to spend my money elsewhere. Maybe if they seemlessly incorporate it into the next edition I might give it a spin, but for now it's an option I can (and prefer to) live without.
3) Most importantly, I rarely have players interested in playing psionic PCs. In fact, the only person I knew who wanted to play a Psionic character was the sort that liked to take the most bizarre character builds imaginable just for the sake of doing so. I'm just not that interested in incorporating gnome-slaying, psionic kobold assassins into my fairly straightforward campaign.