Mind over Matter

Do psionics belong in the D&D game?  It's always been an outside system, something stuck on top of the game in an appendix or supplement rather than right in the middle of the core rules.  Despite the mechanical differences the system has seen over the years, I think the main reasons that it doesn't get used are more flavor driven.  Psionics to a lot of folks doesn't feel right for a medieval fantasy setting.  I think that's part of the reason it saw success in the more post-apocalyptic Dark Sun.  Well that and the fact that for that setting they made a point of assuming it as part of the setting rather than have it tacked on.  Personally, I think psionics are cool but they won't fit every campaign and they need to be incorporated from the start.