Boxers have more purchasing power, for individual expenses.

If we have a 5x Shaman team, and decide to add a tank to our mix.
Versus a one-box player, who has never had a tank and now wants one.

We can buy epic flying, from dailies in no time.
We can also buy Naxx25, Ulduar10, Ulduar25, ToC10 and ToC25 auction house epics much faster.

But comparing my new team of 5.
To a single boxers new single toon.
I have roughly 5x the income, and roughly 5x the expenses.
Quite a few collection quests make my leveling much slower.
But several dailies are group credit down the road as well.