Capes!

[Jeykama]

Well...
1) Mi'qote tails - you saw the outcry the last time they tried to circumvent the clipping for that (Evenstar Coat).
2) People complain enough about hair clipping - if they put capes in, people can complain about hair clipping into not only their hoods, but their capes as well
3) Back-mounted weapon clipping - most armor is pretty good about it, but they'd have to re-animate how weapons bounce on our backs if they wanted to animate capes nicely
4) Funny back-end armors - the i100 PVP robes have some really funky rear ends - having to custom-build a cape animation just for that set would be lame
5) Alchemists have capes?

If it were something simple like Advent Children Cloud's supersleeve cape-lookalike sort of deal with his left arm that's be cool though.