I've always had trouble with this kind of music. I can appreciate the musicianship and quality, but for some reason I can't explain, it's a style of American music that has never grabbed me. Do they call it something? Like West Coast, or something like that? I think its lack of Wagnersque is my problem - I have trouble with it's politeness.
I'm not aware of a name for it. I'd hardly place MM or the Doobies in the "bubblegum" category, though. MM gets called "white soul" a lot. Myself, I hear a haunting dolefulness in his voice. It's subtle, perhaps.
As for the Doobies, I got so sick of them back in the day. The band I was in did one of their covers - god, I can't even remember which one, now. But bands all over did copies so often of their stuff that it was redundant - and boring.
Interestingly, though, I was glad to run into these live videos. I don't think I ever really appreciated how strong they were vocally. I guess that's what comes from overexposure: you don't hear the thing anymore.