Take shamans as an example:

Quadbox shamans is part of a proven Healer + 4xShaman 5v5 Arena team.
Quadbox shamans allows you to recruit just a tank and go instance running.
4xAnything is a nice number for BGs-- 5x is more DPS, sure, but 5x players starts to become a liability in 10 and 15 person BGs.
Quadbox doesn't overly stress a dual-core system, whereas 5x starts to put 3x clients on a single core, which has significant performance impacts.

Of course this will then get another poster who says "heck if you 4 box
then why not 5 box, you are right there and that is where it is really
at etc
I would say that myself if
a.) you had a quad-core system and the other gear to make 5x really workable
b.) you really wanted to excel at PvE instances