My best friend Rob Heinsoo was the lead designer on 4th Ed, and one of his jobs was to fix things that 3rd Ed hadn’t fixed. Multiclassing was on that list of systems that needed work. At one point when playing 3rd Ed, Rob was running a 3rd level barbarian-fighter-ranger. Given the way multiclassing worked, why not?
As the neutral DM I was positioned to see how the opposition to Bob Burman had manifested, especially among two primary players in my campaigns and games, James Goodfellow and Eric Shook. Where does this animus derive from?