Being Navy and having traveled in Army ACU's... I can tell you the two reasons I did it.
1.) It was either the backside of training and I had no civvies, or I was leaving for a 12 month deployment, and I had no civvies or just really didn't want to tote them
and/or
2.) It's the fastest way to get through security without a complete rectum check by a highschool drop out. 99% of the time security calls you around the line and whips your right through.
Navy/Marine rules are different. If you're seeing the blue "aquaflage" Navy uniform they're completely in the wrong, that's a working uniform. Only thing Navy is allowed to travel in is a service uniform of Khaki's, summer whites, or service dress blues (the airline pilot getup), or the cracker jacks for enlisted. Marines are the same, although I don't know their rules on the digital cammies.