I've looked over some threads on this, and plenty of people seem to have an opinion, and it is overwhelmingly against using tank stances, but I haven't seen any numbers to back up those opinions.
Out of tank stance you are taking more damage, or taking less healing than if you were in tank stance; currently by 20% according to the tooltips. So rampart/convalescence will only give you the equivalent of being in tank stance for a short period.
What is the realistic trade-off for making your healers work 20% harder? How much more DPS does a tank put out? How much of a DPS loss do the healers take to make up that 20%? How much shorter is the fight?
It really seems like a middle finger to me to make the healers do that extra work when it isn't necessary. I'd like to see some numerical evidence that running out of tank stance is actually the best option for the group.