It makes sense to avoid gate checking a bunch of bags, which takes time and can delay departures.

I get that rows are empty - I've had a number of flights where the GAs say "we have a very full flight and if you check your carry on we can offer you BG2+" And then the flight goes out with a number of seats and OHBs empty.

I know UA would never do it because they love that tasty checked back revenue, but I've long thought they should allow anyone to check a carry-on sized bag for free regardless of baggage allowance. As it is, everybody without free bags plays the game of bringing a carryon to the gate and then checking it, which is not efficient.