True. Lot of mediocre supports. However you occasionally get the great ones that can absolutely turn the tide. Had a Mercy yesterday who did a whole team revive 3 times on Route 66. Each time they were in overtime, just short of reaching the checkpoint.
And on the other side of things, there's a lot of players who don't properly keep on eye on their support. When you've got a healer watching you back, the least you can do is help them when they're getting harrased by someone they can't fight back against easily.