I would just add the caveat that you won't learn how to multibox unless you actually do it. Sure you can probably boost a team of 4 quicker on your main than you can level a team of 5, but once you start playing those 4 at higher levels where your main can't save you you're going to have to learn how to hit the right buttons.

The only think I have used my main for so far was to run my pally through the Wetlands for his first meeting with his 4 shammy buddies. We'll see -- maybe I'll bust the main out again if I hit some tricky parts.