I think it's pretty simple really- WOW on its own is very rarely mentally challenging or stimulating. Multiboxing requires the ability to multi-task and requires a lot of forethought, coordination, planning etc. A satisfying MMO would be a game that innately challenges a player to think skillfully and logically at all times, rather than being able to predict what is going to happen and being able to automate your behaviour based on that (aka mindless grinding).

There are many times in WOW where you can just 'zone out' or become bored. WOW is conceptually very simple. If you had to be constantly making new decisions, solving puzzles and managing more aspects of character and combat, that wouldn't be the case. I know some people MB because they feel 'underpowered' on single characters, but if it was exponentially harder to play a single character 'skillfully' then a game designer could afford to grant more 'power' to those who were 'skilled'. Just imagine if all the keys you press when MBing were in one window tied to one character, with all the abilities amalgamated.