Stupid teammates aren’t exclusive to QP. Sooner or later every single support player finds their teammate in undesirable or straight suicidal spot. And it falls on them to get their teammate out of that spot, before bad things happen.
It may be not even result of a mistake: your teammate stumbled upon group of enemies trying to flank, by accident.
I only mentioned QP to underline the fact that most players in QP aren’t playing full try-hard mode…and since the OP seems to want to talk about QP, and not comp.
But I NEEEEED 60 hps for when my qp team goes 5 dps (/s). Also, in Mystery Heroes, the most popular “casual” mode, you can make a huge impact on your team as solo healer Mercy if you’re good enough and know how to properly juggle and top off people in the lulls (or let tanks go get mega health pack like big boy and girls). I’ve had entire teams coordinate my assassination because I was contributing too much.
Without an ultimate capable of defending grav or other high damage ultimates, than a team is considerably weaker. Thats why at least Zen or Lucio are in every meta.
It takes shy of 12 seconds to heal an entire mech at 50hps, D.va was taking damage so obviously it was going to take longer to top her off.
Mercy’s beam is the easiest to land since it’s literally lock on, has no resource(so no interruptions at all) and goes through barriers and maintains connection for about a second after you break LoS, which let’s you dip in and out of cover and still heal nonstop. It’s the most reliable method of healing in the game, of course it’s not also going to pump out the most healing and be balanced.
And this doesn’t even factor in Mercy’s GA that lets her follow any hero in this game, even the outliers like Pharah who Moira could only dream of pocketing and of whom Ana is unreliable at healing, at best, due to LoS breaks. Can we at least try to represent her pros and cons honestly?
Zenyatta also has very reliable healing, but no one picks him for that, for some reason.
If you can’t keep DPS alive in 1v1 against other DPS, that’s not one-hit kill, your healing is insufficient and all other “advantages” go to waste. It’s like water pistol: light, won’t be detected by metal detectors, has ammo available everywhere…except it doesn’t hurt, like real pistol.