I think another factor about the negative comments is that some people can't handle other people playing differently from them. It's a sign of immaturity - "This person is doing something that has literally no impact on me, but it is different than what I do, so it must be bad."

Actually, you'll see that a lot in real life... "You spend your money on this? You're an idiot." "You like/don't like that? You're an idiot." It comes from having too much of an ego investment in one's choices - one of the hallmarks of immaturity.

So just don't waste time I do have a keybind that says or replies, "Multiboxing is an acceptable use according to Blizzard. If you wish, fill out a ticket and they will take whatever action they feel is necessary" and then don't give them another thought.