The majority of mma fighters take boxing classes. That's just a fact.
Oh dear. Here we go again. No the majority of MMA fighters don't take boxing classes, it's more usually Muay Thai. This isn't because boxing isn't effective, it is, but because MT stances etc allow for kicking whereas boxing doesn't. None of my fighters have taken boxing, all do Muay Thai and or kickboxing. Most of the other fighters I know are the same. Nothing wrong with boxing at all just Muay Thai along with kickboxing (as well as TKD and karate) are more compatible with kicking. Boxing stances can also be a bit narrow allowing easier take downs.
Even if they did take boxing it doesn't negate my statement that in MMA you can adapt it to make it easier/more effective for your personal use.