He is basically saying that when multiboxers are having fun it is often at the expense of too many other players, and that this is a problem.

"The problem is when multi boxers are having fun in PvP, everyone else often is not." - Ghostcrawler

https://twitter.com/Ghostcrawler/status/322396622594125824

One thing to note is that this is an observation from their lead systems designer, it does not matter if he is right or wrong. What matters is that is his opinion, and he is the lead system designer, and he will not suddently change his mind because someone playing the game disagrees with him.

Multiboxing makes your characters stronger than other players and many players find that unfair. The ingame accepted way of becoming stronger than your opponents is to spend more time playing, divorce your wife and give up a career. Its completely unfair that having a job and income should give you an advantage in any way.

Another perspective is that the playerbase is becoming more and more casual and less tolerant of hardcore gamers such as multiboxers. Its probably a combination, and multiboxers seem to make everyone pissed off.