He took the Doobies somewhere else and commercially it didn't harm them at the time, the old recipe was getting a bit stale, admittedly. I have a live recording of him doing the Tom Johnston stuff - it's credible and his skillful piano playing added a new dimension to their sound, beefing them up live considerably. Some of these arrangements were Toto-worthy.
This is not badly executed music by any means:
I like the fact that the band is still on speaking terms. McDonald still guests (and writes) on their albums for a track or two and shows up at the odd anniversary concert. The band hasn't tried to rewrite its own history. Personally, I prefer the Tom Johnston era, but McDonald's meticulously crafted blue-eyed soul isn't crap.