It boils down to is two more fun for you then one?
Hundreds of players multi-box, but the vast majority of wow players play one toon at a time.

The 15 per month is not really all that much, compared to just about any other hobby.
I five-box, which is 75/mo, which compared to just about any other hobby is basically nothing.

Your kill power increases as you add more accounts and toons.
As you approach five toons, instances become more of an option as a place to level.
Collection quests become more of a pain, particularly in WotLK content.

Through BC, you can level 60-70 without having to do a whole lot of the collection quests.