One of those things you notice when you wear a short skirt is that if you don't carefully guide the placement of the back of your skirt as you sit down, you might very well place your bare ass (or, more commonly, your pantied bottom) on the cold surface of the chair.
I always thought that motion you constantly see women go through when sitting down was mostly for decorum - to keep the skirt in order, and to protect one's modesty - but now I realize that when the skirt is sufficiently short, it has the very practical purpose of preventing one's skin or underclothes to come in contact with the chair (unless, of course, you like that sort of thing).
For a short enough skirt, though, you may well have that problem with your thighs anyway; and for a REALLY short skirt, like a micro-mini (about the length of that pink dress I wore to a costume convention), you'll be placing your ass on the chair either way, because there's just not enough skirt to cover your ass with your legs bent (which is also a benefit for voyeurs when you bend over to pick something up -_^).
