Multiboxing is a joke, and not too many players do it anyways.
Things become gamebreaking when too many people do it and mess up the game for everyone.
Like the Spy addon cheat tool.
They say it was in Vanilla (dunno if that is true or not) and now in Classic so many players use it it totally messes up many players gameing experience!
I think we all agree that if a player use an Addon it should NOT affect other players in any way!
The whole reason Spy addon is a cheat is that it breaks this rule!
Rogues and Feral Druids are forced to run away and stealth, then come back even if you have not seen them the Spy addon cheat tool would have alerted you if they dare to stealth nearby, so your addon is game breaking for Rogues and Feral Druids, Spy Addon ruins the gameing experience for other players and it is in no way a tool ever intended by Blizzard to be used against stealth players and since MANY players choose Rogue, this cheat tool affects a lot of people ruining the fun for them, the whole reason they picked Rogue was probably it’s core mechanic that is stealth!
Like I said before anyone defending Spy Addon is a pathetic cheater.
The same thing is true for Multiboxing, if many players would do it not only a few, then it would completely mess up the game for other players who don’t want to do things in the game that are gamebreaking for others.