I've thought that myself at times, but if you pay attention, the writers do a pretty good job of showing Michael as a competent employee as well. (The Convention - Michael secures a contract with HammerMill; Traveling Salesmen - Michael gives good sales pitch/Andy sabotages; The Client - Michael/Jan take client to Chili's, Michael gets the the client to stay with D/M.) Michael is ridiculous, but when it comes down to it he nearly always gets the job done.