They go home disappointed that no one was knocked down or knocked out. But hey a 12 rounder wherein someone wins by a SD or UD is very beautiful to watch. You get to see some skillz. You really see the beauty of what this sport is all about. You see why they call it the sweet science.
I'm not entirely opposed to it. I've just been to a fair number of local fights where the co-main eventer or main event (Ronald Hearns comes to mind) fights a guy who looks like he wants to be there, then flops on the ground or gets blown out. It's frustrating.
I don't mind ordinary main event if the undercard offers me plenty of good moments. I don't pay to see one fight, i pay to see multiple fights, so that way all the pressure to be entertained is not all on the main event.