Alt-itis is what caused me to get into multiboxing in the first place. I found out that I could use my 70 shadowpriest to help my 60 warrior level and quest! After that, 5boxing came... crazy stuff. It does worry me though, because I have lost all interest in end-game raiding (mostly the harsh schedule that it presents). The truly scary part is that I'm now 70 on my shaman team, I have no interest in leveling ANOTHER group of 5, and I wonder if PVP is enough to keep me interested in the game. If not, then I may just drop the game in a month... we shall see.
Leveling is 90% of the fun for me.