There would be no Mike as we know him if he was on mic right? The whole Mike persona was something Tom could invent because for years no one knew what he sounded like or if he was even saying half the stuff Tom attributed to him. You also don't get funny bits like Tom hanging up on people by insisting Mike is making him do it or the whole call screener Steve saga. Like other's have said the whole mystery of Mike is what was so funny, at least to me.
also, just logistically Mike isn't even in the studio, he's in the other room presumably doing his job most of the time. What're they going to drag a mic out there every time?