Personally, I'm against an in game parser as it will lead to people being more vocal towards those who play sub optimally. While this isn't always a bad thing (as some people are courteous and helpful while being vocal), there are also those that are just plain toxic. I have never seen people harass others personally, but I do know it happens.

For those who claim that SSS isn't working because clearing the SSS doesn't tell you how well you actually do in the fight, I call BS. If you are skilled enough and know your rotation well enough to clear the SSS requirements without completely outgearing the SSS, then you know when you are under preforming in the actual fight. I can clear the A8S SSS at i222 as a BLM (1800 DPS required for BLM), so I know I have a pretty solid understanding of my rotations and proc/cooldown usage as a BLM.

I've only cleared SephEX a few times and I know without using a parser that my dps is low for that fight. I know this because I know how to play my job. I know that I'm not completing my rotations properly in the fight due to having to move. I know that sometimes I use a proc too soon instead of saving it for when I do have to move. I know that I messed up by not moving into position sooner by slide casting my way there and thus am losing out on a GCD which adds up to a lot of missed GCDs by the end of the fight. All of this I know not because a parser told me, but because I know my job and I know when and where I made mistakes. I also know that every time I go in there, I improve and get better. I don't need a parser to tell me this either because I know that each time I make fewer mistakes and I get better at prepositioning.

This is true of all content and is true for all players who truly know how to play their roles. For people who are still learning their roles, they don't need a parser to tell them they how they are performing or if they are improving. We already have a tool that can tell you that which I mentioned previously, the SSS dummies. Pick an encounter appropriate for your ilvl and go from there. If you failed horribly, then you know you need to improve and will seek the information required to improve. A parser isn't going to make you better just because it's telling you that you have low dps. Only the player can make themselves better by seeking the knowledge required and applying it. If the player doesn't want to take this extra step, then a parser isn't going to help them anyways.

I use the above method for every job I play in order hold my own if you will in any content I choose to bring that job to. At the very least, I read all the guides and the various theory crafting discussions for every job I have at 60 so that I have a firm understanding of that job and their rotations. Because I do this, I'm able to assist players that are open to criticism and are willing to improve that I run into during a DF run. I don't need a parser to tell me if they have low dps, I can tell by looking at their cast bars or character animations and buffs/debuffs if they are preforming their rotation properly. (As a point of reference, I have every job at 60 except 50 MNK and 57 DRK).

TLDR: Parsers aren't needed for a person to improve, knowledge of their job and a desire to improve is needed for them to improve.