My view on why to apply early:
1) If you're a median applicant they're more likely to offer you a seat when they have plenty open.
2) If you're an applicant that exudes "I REALLY want to be at X school" (and thus much more likely to accept an offer) with below median but acceptable numbers they're more likely to offer you a seat when there are more available.
3) If you drop apps in early you're much less likely to be waiting until April (or June... Grrr Davis 2007-2008 cycle) to hear back from at least some of the schools you'd consider going to.
4) If there is a problem with your app (like forgetting an addendum) better to have months before the in-depth review to find out then days.