The best part with Multiboxing is

Bored and we need such a tread to convince others to start boxing! :D

I will start:

The best part with Multiboxing is that you can cheat on your dueling stats!

The best part with Multiboxing is you don't have to deal with idiot pugs.
The best part with Multiboxing is you can farm a crap ton of gold from dailies alone.
The best part with Multiboxing is you can do a ton of dps.
The best part with Multiboxing is the power of focus fire. <--- DO NOT UNDERESTIMATE IT!!!

the best part of multiboxing is ability to tell apart idiots from normal players through forum whinning and being called "cheater" LMAO

The best part about multiboxing is that you can choose to create 5 hot ass female blood elves/night elves that can do everything you like.

And 5v5 is more fun...

The best part about multiboxing is i always play with the best player in the world! me! :)

So you can /ROFL and then /hug after killing the lvl 69 DK that tried to gank your lowbie lvl 58 frost mages.

edit: it was lvl 69 dk. ooops. :)