Wouldn't that also render your account essentially useless when your trip is over? (Unless you're traveling for, what, years at a time?)

No, again, the point of the travel lock isn't that you can't use Facebook at all, but rather than your history and social graphs are restricted. For most ordinary purposes, Facebook will probably get more usable while travel-locked.

Yeah. Everyone* would be able to see what I’m posting during my trip (which itself could be dangerous…), but no one could see the history until after the trip.

* who is normally allowed to see what is posted in the first place.