Because why would you go over to a 1 v 1 with Soldier and Moira when he still has Biotic Field, his health likely wouldn’t drop to critical.
By the time Mercy comes over, Biotic field is gone.
So it is 50hps against 50 dps. Moira still stands a chance at healing enough from Biotic Grasp if she moves around well, enough to throw an orb and get the kill on Soldier.
Moira doesn’t necessarily have the advantage, but in a 2v1 like that, she isn’t at a disadvantage either. The point I am mostly trying to make is that Mercy stands a chance at failing to pocket in a 2v1 situation against even the lowest damage dealer if they throw out a little extra damage.
We want to say that she is a pocket healer, but a pocket healer wouldn’t suck that hard at the job that is supposed to be the only thing they have left.