No one's saying that they shouldn't get credit in their own right. What's special about Patton is that he was so well-accomplished at many vocal stylings, such as growls, crooning, screaming, and rapping. He brought so many elements of different genres into his work. Is he overhyped? Well, sure. But it's immature to disregard him completely and his contribution to music.
Oh, and for the record, don't take me as a fanboy. I'll be the first to admit: Mike Patton is a horrible songwriter. The only good albums he's made are when he aligned himself with great musicians. Just look at Faith No More's post-Martin output. It's bloody awful.