Well.. There's a few answers to this. And it keps coming up. On the RP servers at least, I'll leave you with an opinion I received from a female friend, "most dudes that play chicks do it to ICly hit on chicks (in RP Servers mind you), and they do it because they couldn't ICly hook up with the same chicks on the dude toons, so they made females. Now they're all dudes playing chicks who are doing chicks that are played by other dudes."
That response makes me laugh every time I think of it.
On a side note, if its on a PVE server, or is just a purely PVE, non RP toon... Who cares?
If is o an RP toon.. hey, its their gaming experience, unless they're trying to trickdudes into dumb stuff.... So again, Who cares?