I am rather of two minds on the subject. I've never felt like psionics were entirely out of place in a fantasy setting, but I certainly don't feel as if the setting is missing anything when they are absent. I tend to adhere to two basic rules in my games...
First, allowing that Psionics have ever been presented as an "optional" rule in D&D, at least since the 2E overhaul (prior to which they represented, in my opinion, a serious balance issue), if none of my players elect to include psionics in their character construction, I tend to ignore them, not working them into the game at all unless I come across an idea I just can't resist. Of course, if a player
really wants to work them in later on... and he/she can propose a
very convincing argument... I'll work them in. But groups with psionic characters in my campaign should certainly expect to encounter psionic villains.
Secondly, I enjoy the feel of psionics more when they remain rare and fairly low profile. As mysterious and enigmatic as magic can be, psionics should be even more so... after all, magic seems an integral part of the fantasy mileu... not necessarily the case with psychic abilities.... IMO.