I'm still 106 so have only had glimpses of the 110 gameplay, but a lot of people say mage pets are really strong. However I briefly grouped with a guy who had the Golden Sage's Earring which is one of the highest pet focus item (I think only 1 better from tough raid?) so his pet was as good as it gets. We killed some nameds in RoS and it did ok with a dedicated J5 healer merc. But he said it struggles against some nameds and on bad pulls, and for some situations he gets some friends with their healer mercs so he has 3 healer mercs to keep the pet alive... In other words, it is good, but a real tank is better. And if the tank has good gear and a lot of AAs, it is a LOT better. I used to run with a mage pet as tank and it worked fine for a long time, but against multiple mobs it was bad because the pet had no ae taunt. For that reason alone, I changed my party to have a real tank and I am glad for it all the time that I play. On tough pulls I can survive because my tank hits ae taunt(s) and a strong disc, and it saves the entire group. Yeah I have a bard that can ae mez but it takes him a while, especially if there are some resists. And against easy mobs I can pull 10 mobs at a time and gank them down one by one while the tank holds them all.
I am sure you could get by with a mage pet though, but would need to be careful with pulls, go slower, and ideally have 2 healers.