It's standard operating procedure for a Psion to take Psicrystal Affinity at first level. According to the XPH and SRD, "This feat allows you to gain a psicrystal."
By RAW, and by my reading, it would seem that you don't have to keep the feat to keep the psicrystal. You only need it to first create/shape/bring into being a psicrystal.
So, in an optimized build, why keep the feat if you don't have to? I've never seen this addressed, so I'm bringing it up now. I'm fairly certain the reason the feat is kept is because there are no other feats (that I can think of) where you can divest yourself of the feat and keep the benefit.
That said, the one benefit of keeping the feat, again by RAW, would be that if you lose your psicrystal you can gain another immediately. The text never says that you can't gain another psicrystal, nor does it impose any time constraints on the regaining of a psicrystal. Heh...of course, unless I'm mistaken, the text doesn't say you can't have multiple psicrystals, either.
Anyway, can someone tell me if there's a reason given in the rules that makes it necessary to keep this feat?