I get why they do it.
Closeups.
The crotch area on the basic UV is tiny. It just doesn't support a lot of detail. Mind, It's enough for most rendering, but a close up is going to show the lack of pixels, especially on the extra geometry in the gens.
What would make sense, from my POV would be some like the New Gens do. One UV zone for the genitals and the rest using the base UV. I know that requires a script to blend things, but all the scrip does is invoke a LIE preset, and that would be easy enough for anyone.
Maybe I should do that...