It's not Mike's VOCALS that are annoying me personally... I would love it if he rapped more verses.
Of course I acknowledge the fact that I do not know the band or their exact roles, however, this is how I interpret the situation -
Mike is an extremely talented person, one that I would consider a genius in his level of creativity. However, he is trying too hard to be good at everything instead of mastering a couple of things. He is an incredible rapper, but he is not adding to that anymore. He can play piano and guitar, but are not improving at those either.
Singing is a natural talent, one can hit the notes with perfect pitch and still not be an incredible singer. To me, Mike is an average singer (he's not bad, but there is nothing that attracts me about it), but he is unwilling to accept the fact that he is not an excellent rapper/producer/keyboardist/guitarist AND singer. He'll have to accept the fact that Chester is better eventually...