What awesome pics everyone is posting! Especially that static - very pretty.
My theory as to why belly patterns / colors aren't as big a thing as other elements of a BP's body is because it's literally the least visible part of the snake and in most cases, you have to stress the snake out and make it do something that it most likely doesn't want to do to see it (ie flip it over, hold it weird to get a good pic, etc.). With a good top pattern or crazy head pattern, you can see it 100% of the time without bugging the snake too much versus seeing its belly pattern maybe 5% of the time.
Anecdotal account: I have a pastel YB male that has the most awesome checkered pattern on his belly. I've had him for about 6 months or so and only flip him over to check on his scales and make sure everything is healthy - he usually corrects the flip pretty quick too. Other than that, I don't see those cute checkers as he isn't too keen on flashing his belly around for everyone to see.
tl;dr Cool belly patterns/colors are a bonus on an animal but aren't usually a selling point (unless it's aiding in identifying the morph as genuine) and won't be a predominant selling / buying point for most people in the future me thinks as it's not as visible all the time as other parts of the snake. Of course I could be horrifically wrong; just my possibly inaccurate prediction