What team are you running with or what problems are you encountering that you are having problems with at end game in WoW? Many people have progressed through Heroics and more and more are boxing raids all the time so WoW can definitely be multiboxed in end game. I would imagine that all mmo's are going to be difficult to box when you first reach end game but will progressively get easier as you gear up and improve upon your own personal skills.

I know for me I rushed to 80 when wotlk first came out with my 1prot warror and 4 ele shammies and was super dismayed when I saw all the differences between wotlk heroics and bc heroics. But now that i've gotten use to alot of the mechanics I can knock out most of the heroics very quickly.