The debate is usually based around PvP, with very few people worrying about how it affects PvE. Some people do try to find angles for criticizing its use in PvE but these tend to be weak. The whole "this isn't how the game was intended to be played" bit is an example-- there are many ways in which players change the way the game is played, and as long as they aren't breaking any rules it's possible that the designer will incorporate those player-driven changes into the game. An example being the way enhancement shaman off-hand weapons are itemized. Shaman found that slow weapons were better for DPS, and Blizzard has added slow off-hand weapons to the game.

It goes the same for most other PvE-related arguments.

But if you notice most of the complaint threads on the WoW forums, they usually are started by someone who got ganked 5-on-1 by a multiboxer and they're very angry about it. Most of them are upset because they figure they just got griefed by a cheater or bot, while others understand what happened and are angry over the unfairness of one person having five characters at their disposal. But once you work past the anger, most of their issues are also pretty weak and require that they bend and twist facts and logic, sometimes a lot.