I agree with Merc, I've been running one of my sorcs with a tank pet and he's only barely better than I am when it comes to soaking damage. Even spec as a healer, spamming heals and shields the entire fight my "tank" pet still ends a fight under 50% health.

I also have another healer sorc that I run with my husband, who is a tank. We were forced to run dual tanks for a while {him + my tank pet} and it was a nightmare. Killing was slow and again it was spamming heals on my pet. I finally got my DPS pet and I almost never have to stand around healing.

Unless your idea of "tanking" is throwing taunts. In that case, sure, they throw taunts like a champ.