I can definitely see where fulltimers and snowbirds, etc., would always use their own. That just makes sense when you're living in it as a home.
I think the differences between the short trip users is more about their particular setup than anything else. Small cramped shower stalls, small water tanks, etc., may have folks enjoying the campground/park facilities. Full hookups with a stall you can stand in will encourage using your own.
As for the "germs", I always wear rubber flip flops in a public shower - even as a kid at camp. Keeps your feet off the floors until you can dress and dry off your feet and put socks and shoes on.
