If there is a chance that you do not match the residency of your choice in the military, and you do not want to wait around forever in the military trying to get that residency, you may want to put in for a civilian match and do a deferment. In that case, if you are applying to a competitive allopathic residency, having taken the usmle may be to your advantage.
I am just trying to play devil's advocate. Really, if you plan on staying in the military for a while, taking the usmle may be totally unnecessary.
I have a friend that used to work in dermatology as a tech at NMCSD San Diego. I had him ask some of the derm docs he was working for about this very question. One of the captains he asked said that without a doubt, if I am even thinking about derm in the Navy, I need to take both the COMLEX and USMLE. Nobody knows for sure if this guy really knows at all what the selection process entails, but it still made me think...