Let’s put it this way: being support means trusting your team to get things done. And trust isn’t something you normally give to 5 strangers.
More accurate would be “worth” anything. Like DPS aren’t achieving anything by shooting wall and not enemies, supports and tanks aren’t achieving anything by supporting teammates, who are not getting things done.
It’s also one of the points, where medals clash with victory: to win means healing capable players, to earn medals - heal player taking most damage.