Unlike other parts of the world, many airline pilots in the US "commute" to work on the airlines. Wearing their uniform is also required by some carriers to jumpseat, even if they end up getting a seat in the cabin. It also makes it much easier to deal with security while in uniform, since employees do not have boarding passes to get through security, but jumpseat forms that are issued at the gate.
However, with that is mind, I know too many idiots who refuse to wear their uniform when in the airport enviroment. This same group is trying to relax the dress code by wanting hats to be optional, polo shirts, etc....
When I was commuting several years ago, I jumpseated on a large regional airline on the west coast and was absolutely shocked to see the pilots wearing hideous polo shirts and baseball caps. Apparently it was managements' idea to keep everyone happy during a union drive that lasted a month or two.
I think it relfects poorly on the profession when one cannot wear a uniform proudly. It commands respect!