I think the reality of Bezels is this:
Mame users, first and foremost, want original artwork (Bezels). No matter how nice/good custom artwork is, the original artwork wins out. This makes sense when you consider the stated goals of Mame itself, and the amount of time and effort Mame users put into getting scanlines correct, Mame cabinets authentic etc. Even aesthetically inferior artwork - if it's original - beats out custom artwork.
As you state, BK, there are literally 100's of games without a Bezel of any kind. These allow for creativity, and add to the overall Mame experience.
And a personal note: I love Bezels. I think every game is improved with nice artwork. However, my priority is the GAME. When designing/making Bezels, it's worth considering that players aren't likely to sit there gazing lovingly at the artwork, they're going to play the game itself. Given this, priority with regard to screen real-estate should go to the game. In short - the gameplay area should be maximized. This, logically, limits the dimensions of the Bezel. I'd of loved to have had a larger top and bottom area to work with in a 4:3 game when doing Bezels - but to do so would take away from the primary reason people use Mame - to play and enjoy the games. The number 1 reason I'll play a game in Mame and NOT display the Bezel, is when the Bezel makes the gameplay area too small. This happens quite a bit with official artwork, and is an example where Mame can't quite emulate the experience of playing the game back in the day. This is because the real experience was three-dimensional, Mame is two-dimensional - it lacks depth. Regardless, speaking personally, game first - artwork second.
Just my 2 cents.