I recall that [they were deleted] because the pools were becoming just a second tag for their contents; that is basically any approved image with an exposed ass in it was in consideration to be "good enough" to go in the Perfect Ass pool and it began to cause problems with users fighting each other for how high a standard the pool had for entry.
The ones that remain were left 1) to appease the whiners and 2) because they represent things that are harder to draw accurately than a couple semicircles (exaggeration, but still).
Basically, they turn into pet pools a little too easily.
If you think an image looks really good, favorite it and maybe say something about it in the comments section. It doesn't need a special label otherwise.