I don't think it is a question about whether checking mixes in mono is a valuable tool or not, I think we all agree that it is, it's whether a mix needs to sounds good in mono or not. In my opinion sometimes you reach a point where you have to say "you know what? this sucks ass in mono but it's not relevant to the majority of my target audience". I also think that metal guitars usually sound shit in mono. The more guitars you add the worse it gets. I also think that metal listeners are more likely to have better listening environs that other genres.