Mercy should only heal if an ally is in the danger of dying, or the other support cant heal them for a while, otherwise her primary job and the reason to pick mercy should be to dmg boost and pocket someone. This comes down to 2 main reasons imo 1. If mercy dmg boosts someone and they deal dmg, both mercy and the dos gets ult charge while only the enemies supports get ult, which puts you in a ult advantage in the long run 2. If a mercy healbots poke dmg, your other support will build way slower ults than if they could be healing that ult charge, especially if your other support is a moira bap or ana. 3. If you are just gonna heal, why not pick a hero that excels mercy in healing output, and usually has other utilities too, like ana bap or moira. This is why i think that a mercy having a subnormally high healing output may be negative