I think the ads might be the reason some fighters wear the loose shorts, but I think the main issue is of personal preference.
Personally, I would wear the tight shorts because they allow for better movement, etc., AND a fighter can't grab onto my shorts while the ref isn't looking. I guess it's more personal.
favorite MMA fighters
A. Rodrigo Nogueira
Mauricio "Shogun" Rua
Georges St. Pierre