When the alternative is to go out with those F seats empty and the next pax down the list left out because they held onto them for no-shows, I definitely prefer the GA eventually give up on the pax who didn't show up to claim the u/g.

If I'm waitlisted and it looks close, I definitely go to the gate early, check with the GA, etc., and on at least a few occasions have gotten a seat due to someone not showing up.