The issue with this mindset and heroes that are designed this way is that it encourages 1 tricking.
Players that can ONLY play Mercy or heroes like Mercy and nothing else.
So if you have 2 Mercy type players in a game and 1 chooses Mercy the other is lost and cant play anything else at a decent level
As opposed to support players than can play multiple supports. Most high ranked support players that Play heroes like Ana, Bap, Lucio , Zen, have no issue playing Mercy at that level as well.
But the same cant be said for a strictly Mercy player that tries to play Ana, Brig, Lucio, Zen, at the same level they play Mercy.
In short: it encourages 1 tricking and it makes it so you limit yourself, not leaning any other heroes with different and more difficult more rewarding mechanics.