The more I play Mercy, the more I think she needs some tweaks.
I almost never use the damage beam anymore, except when I think there's a second or two where the patient has the element of surprise and definitely won't need health regen. 50HP/s is so powerful that I can keep a Bastion alive while half the enemy team's shooting at it, whereas if I just boosted him, he'd die immediately before I could switch back to HP regen. And because I stay on HP regen so much, I very rarely actually have 2+ teammates down to use my ult effectively. I'm thinking maybe it's better to actually boost someone when 1 teammate's already down, let that person die, then use the ult and go back to tanking someone.