DPS display could easily be solved with a system already in game.

[Zohar_Lahar]

If "bad DPS" is caused by inefficient use of rotations, then how is a vague number from a parser supposed pinpoint the problem?

[Vire]

If "bad DPS" is caused by inefficient use of rotations, then how is a vague number from a parser supposed pinpoint the problem?

Look at a website like world of logs for wow. You can analyze peoples rotations, cooldown timers everything on comparable fights and figure out where you are screwing up by isolating specific times during the fight on the graph. This game does not have anything near as robust yet because parsers are not allowed. They absolutely help thats not even up for debate and if they ever made an official one you could expect comparable tools to be developed for this game.

In addition to that tools have been developed as well for wow like simulationcraft which could tell you the exact expected dps on a fight with your current gear and what you should be expected to put out.

[Zaft]

If "bad DPS" is caused by inefficient use of rotations, then how is a vague number from a parser supposed pinpoint the problem?

Because it's not "vague numbers". The number is just a summary of the overall amount. This is what you get from real parses.