Giants takes too much place and have a too high ECL. Mindflayers ? Too high ECL, and not fun to play -- someone posted about his mindflayer PC and how it was uninteresting, and how gaining a level of psion or wizard wasn't at all good, while gaining a level in another class seemed silly or counterproductive (like fighter for a character that avoid all melee whatsoever, if need be by planeshifting away). Mindflayers are not for races of faerûn, but for monsters of faerûn (which, indeed, contains the alhoon).
Poor flayers, they are spread everywhere: base monster in the MM, lichs in MoF, their pet and a monster in the Psi handbook, the Elder Brain in the upcoming Underdark, and brain golems and ceremorphised creatures in the Fiend Folio.
Centaurs and wemics are much more playable.