I use my main (70 lock) to level my team all the time. Keep them slightly behind and SoE packs of mobs or drain tank bunch of them. Whenever it gets a bit boring to do the normal leveling with my group, I jump on main and do an hour or two to get some fast xp.

Don't see anything wrong with that. You're using your main to level your alts.. it breaks the monotony between pure questing/grind for some dungeon runs and some good loot. Though you'll find that as you get higher you won't be able to clear the whole instances with just your main. 30/40 seems to be the sweet spots for 70 solo clears.