Unpopular opinion warning.

Time and again players have shown that if there's something they can lord over each other they will. If it can be used to put someone down, ostracize players, or otherwise make the overall play environment more hostile and unfriendly the playerbase will find a way to do it. Any game, not just XIV. All you have to do is look at the responses in any thread about this topic to see it. It's the reason we can't have nice things, because people can't be trusted with nice things. These imaginary players everyone's talking about who are playing incredibly poorly won't improve just because there's a way to measure your damage numerically in-game. Players who want to improve will naturally seek out discussion and practice to improve, with or without a parser.

On top of that, many players misuse and misunderstand the data provided by these tools, or tunnel vision on DPSing at the expense of mechanics because they're chasing a meter. Not having a meter doesn't stop people from doing this either, but if there's an easily available metric with which to competitively measure yourself against your peers you'd best believe you'll see more of it. Not having a meter also doesn't stop a player from working to improve, especially given that there IS a tool in game to measure yourself against time-wise.

Call me a cynic, but I've seen enough damage done over the years whenever players get their hands on a new shiney toy to trust that kind of thing becoming widely available.