And btw, I agree with the party being equally responsible for clearing an instance, but doesn't that just mean that everyone wants to do their best to succeed?

Let me use an example. A certain fight has a strict 5k dps check for dps players alone. We have 3 dps player doing 1,3k+ dps. We have one dps at around 800. We'll be missing about 300 to clear the check. Would you rather ask the people already overexerting to do an extra hundred, or ask whether there's something the fourth one can do better? Or would you disband the party, get the low dps again, and keep bashing your head to the wall?

Personally I would hate to be the fourth person and not know I'm underperforming, but that's me. Most people would be happy to get carried through everything and anything.