Before my local track ran bump-ups, half of the guys in the lower mains would either leave or just not run the race to focus on the other class that they did make the A-main in, which made it very lame for the other racers and any spectators that may have been there watching that night. Now that there's a race for the bump spot every week, everyone shows up and we give ample time (at least 3 races) between the B and A so the car can be turned around in time for the A-Main.
That alone gives me reason to support the bump-up system.