A grade system with rewards attached to "being good" is not good design for a cooperative game.

You should not reward "good players" from "playing normally" or push players into "playing better" for the sake of a cookie. If we win, we all win together. If we lose, we all lose together. This is what a team means. Any team will have stronger and weaker players. That does not make it any less your team. As a player, you can either deal with your team or give up and find another team that better suits your style. A grading or ranking system will encourage players to put their needs (their grade) above the team (success of the mission). If the reward is meaningful, when a player knows he will get a bad grade due to the team, why continue? Why risk losing the good grade to help a player in need? Damage dealers have long queues, so they are less likely to give up too quickly, but a tank or a healer?

There is one real underlying issue going on, but it is not what the majority thinks. A player should never think about enforcing his playstyle upon other players. The real question is, why does it matter? If we go purely on a numerical damage output value and assume that every player are able to fulfill all the mechanics. Assuming that everyone has the adequate gear for the challenge. Assuming that encounters are failed due to a lack of damage output. Let say that one player doing bellow the needed damage is enough to make those encounters fail.

We have established that this player knows mechanics, has the proper gear, but still cannot provide the necessary damage to overcome the challenge. A grade system will reveal that this player is not good enough. A parser will reveal that this player does not match the numbers of the other players. Neither of which will allow the player to improve. Repeatedly saying: "you are bad" to a player will not magically transform this player into a good player. All it tells the player is that his or her playstyle is inappropriate.

This is a symptom of a bigger problem. Why is the gap between a "bad" player and a "good" player so large? There is a very simple solution to this damage output problem. It is to give each job an easy and effective pattern of attack that will do enough damage to meet those damage checks. Think repeatedly pressing 1-2-3. Make this pattern do 70% of the maximum damage of the job. Using the complete toolkit properly will make you go above 70% or lower if you screw up. The better you are, the closer to 100% you can reach. This is a design choice used in many competitive games (fighting). There are move sets that are intended for novice players, which can give them victories just by button mashing. This stops working at higher level, because advanced players are able to counter those obvious moves. Even if they are capable of countering the moves, they remain affected and if those advanced players lazy out on the fight, they can bite the dust. Once you start improving and get a better understanding of the game, you move on to the advanced move sets, but until you master them, you are vulnerable to the button mashing techniques.

Button mashing, or in a MMORPG, only pressing 1-2-3 is a boring strategy. For some players, that is all they want and they will be satisfied with that. Others will want to do the advanced techniques. It is actually much easier to ease players into playing their job properly if they have a safety net. If they are unable to use the complex abilities and their damage output starts dropping low, all they need is to go back to focus on 1-2-3 (or be advised to do so). Because this 1-2-3 will inevitably part of their rotation, their damage output will only be so low. Let say the worse they can do if they really try and fail is 50%. This would make them, at worst, only at most half the damage output of the best damage dealer of the party. This also means you only have 20% missing damage to compensate on the other 3 damage dealers.

All you need is an easy way for any given job to perform enough damage to meet the DPS checks, and all that is left are mechanics and gear. The greatest strength of that system is that a player can adjust on the fly to use the easy rotation. This means that you can effectively transform a player that is a burden into a player capable of carrying its weight mid-fight. No grading or parsing system in the world can achieve this. Ever.