What's the point of Mercy?

That was indeed the period before Mercy gained the Invulnerability buff, and then it was useless.

Here: "Only bad Mercies hide & rez"

Plenty of videos etc that showcase it in GM/top 500.

There were Mercy players that climbed to GM using the exploit, so saying it only happened in lower ranks is false on it’s own.

Now if you please answer the question to your idea.