It's not a mess dress, it's a service dress, uniform. They are worn when one wants to not look like one just came in from the field. The current practice of wearing BDU's, ACUs, or whatever for all occasions, bothers me somewhat. The Army is the worst about that. The Marines don't do it much at all, I must hasten to add. They have to wear the Alphas when traveling by commercial air for example, and they where them for meetings outside of military bases and/or with non military personnel, such as contractors. The Air Force is somewhere in between. The Navy is ... well.. the Navy, with more uniforms and combinations than the Army and Air Force put together.
A couple of weeks ago Jay Leno had an all military audience, only the Army guys were in utility uniforms. (Desert Cammo). The Air Force was in Service Dress, the Marines in Dress Blues, and the Coast Guard and Navy in uniforms comparable to the Service Dress of the AF or to Marine Alphas (not as fancy as the Dress Blues, which IMHO are the sharpest uniforms of any service.)
"It's not a mess dress, it's a service dress, uniform"
I realize it is service dress/class A etc...never/very rarely used...
I have yet to wear/own a mess dress....