Personally, as a DM, I have found psionics to be extremely tedious and good for slowing down gameplay. In fact, it's gotten to the point where I just outlawed them all together. Maybe it's just me, but I find that it takes an awful lot of work on my part to keep up to date with psionics. I prefer the core rules having only magic/melee abilities with psionics being an add on that individual groups can choose to implement or not. I believe that putting them in the core rules would make for much more complaining by my players when I tell them that I don't allow psionic PCs in my campaign.