Some theories:
1) He's spread too thin.
2) Needs the income to finance his fledgling network.
3) It's a holdover from his "morning zoo" days.
4) He needs a break in his day.
5) People like it.
6) It fills a conservative niche.
I initially felt the way you did, but I've gotten used to it. Some days I need to take a break from Levin and Rush, or at least need a humor break.
His best show was after the 2010 mid-terms. Pat and Stu were hilarious, celebrating the Dems defeat.
But I can understand the frustration.
People usually tune in to hear "the man." Not his side kick.
That's always been my experience anyway. I used/was a co-host for years but my best radio for over ten years I did as a solo act.