Having players deliberately or not deliberately underperform is and always will be bad for group content. But I prefer those than toxic trolls. A bad player can learn. A troll can (will) not. I've seen it numerous times in this or any other games, what happens when two (or more) toxic players start with their daily dose of "mommy, my nose is runny again" cry for attention. It can derail whatever the group was doing like not bad player can.

It's very simple, really. One should always ask oneself "Would I have the character, desire or courage to say "you are bad" to this person if we were face to face?" Meaning in real life. If the answer is "no", you should just forget about it and move one. After all, you don't want to be one of internet trolls, do you? Having courage to insult others only when hiding your true identity.
Who needs people like that? They are just BAD.