Here is HQMC logic:
"Our Marines have to purchase new uniforms regularly, so we will give them an additional once a year bonus to help defray the costs of said uniforms, and everything adds up."
Here is PFC logic:
"My sweats are barely even ripped at all...Beer money it is!"
Edit:
Not that it matters in the slighest, but my $.02: Stick w/ green on green.
Cheap, Versatile (worn with cammies), Practical (pajamas in the field), Tactical (Can wear a reflective belt when needed), and Lightweight (seabag factor).
When I go to the gym on base I don't particularly care if everyone is wearing the same clothes to exercise during liberty.
Short of a flashing neon sign saying "UNITED STATES MARINE," I don't think average civilians are necessarily going to know who is running in formation, or why people would even run in formation. Joe Six-pack doesn't know what an Eagle, Globe, and Anchor is and probably wouldn't recognize it if worn on the back of a PTing Marine. The high and tight haircut would probably be more of a giveaway - or the fact that you are probably right next to, or inside of, a Marine Corps base...