The best part with Multiboxing is

[Dominian]

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!

[Zal]

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!!!

[Dzonatan]

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

[Negativ1337]

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...

[mooglej]

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

[blast3r]

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. :)