I believe there are 16 parts because you can bounce style tracks (just a guess)...and if that's the case, they will have to share the same EQ/effects, which may or may not be advantageous.

Personally, I find 8 parts as complex and full as I would need...especially since Yamaha uses two drum tracks as opposed to Roland's one...

I sometimes take out style parts to simplify, but I guess if you're only playing single note lines over the style, 16 parts might help fill things out better.

Ian