The XP will be cut down, but you will kill things much faster, thus it's a good idea.

You can usually power level to about 40ish, some take it a bit higher.

If you want, try this method for super XP.

Gather a lot of mobs with your 70, then run to the entrance, and dot them all once (for me, in SM cath, it was 1 corruption) down to about 30-40% life. Then, when they are all that low, dot them all once more, and jump out of the instance before they die. This will make it so your lowbie alts get all the XP, like they just soloed a lvl 35 elite.

There is a lot of specifics to it, like hiding your alts, so that when you zone out, the mobs are far enough that they dont run over and kill your alts, before the mobs die.