I'm not saying he's impervious to the affects of crits, that's the point of why it breaks gameplay. We're talking about random crits here.
A skilled player doesn't stand a chance against another who random crockets you in the face as you turn the corner. There's no 2 ways that scenario could have occured, the crocket was fired and it was impossible to dodge, and it one-shot kills the average class so you died. But because there's no way of governing random crits it's also inaccurate to state it benefits any particular player or team all the time.
Getting random crits only makes you "useful" when you get random crits, which has nothing to do with you other than the fact you're in the game clicking the shoot key, and these crits don't necasserily always hit and affect gameplay. It's not something to be relied on. It just happens and it is annoying. Even killing people with crits annoys me because there's no challenge in it. It wastes perfectly good combat experience.
A skilled player complains because they had no chance of reacting to the threat. Atleast if he got shot 3 or 4 times in the back he could admit he was not able to realise the threat in time and it was his fualt for not being more weary, and learnt something from it. A crit is just another death tally and 20 second respawn wait for no reason at all other than the 'game' decided it to be so.
It's about as fun as having a random player in the server executed every 30 seconds for no reason. Amusing to watch from a distance but ultimately damaging an otherwise anjoyable gameplay experience.