I'm in the same camp. Out of sight, out of mind. A locked panel is something most people will pass by unless they have a grudge, they are intoxicated on some substance, or teens being teens... and are in a mood to trash something.
I don't consider Europeans paranoid, but all their hatches (fresh water, gray water, even the door for the cassette toilet) are all lockable.
It also gets the ports out of the weather as well. Even though a FW inlet may be capped, it isn't unwise to have a flap just to keep the mud and other road grime from being there.
Paranoia is worthless... but there is always having at least some resistance to attack, and even if the panel or door is opened by a CH751 key, it is far better than nothing.