An in-raid parser is pretty worthless. In other MMOs, where all damage dealers are expected to do the same dps, yeah there's value in it.
But here, not all DPS is equal all the time.

By giving us a method to determine if we're capable of doing the damage (having the gear minimums etc) required to clear specific content we know that yes, we are in fact capable of doing the damage to kill the boss. The question then becomes "why can i not do it on the boss?"
You then become more attentive, try new things and generally learn the encounters to do the best damage you can under the circumstances.

Having an in-raid parser would only show you numbers. Numbers are meaningless without context.
Figuring out ways to maintain maximum damage during boss phases should never require a parser.

As an example, my Monk in Vault, first boss. I could never keep my GL stacks up initially, but doing the dungeon more and more and actually paying attention allowed me to get to a point where today i can now maintain GL3 between phases and various attacks for the entire duration of the fight.

Trial and error doesn't require a parser.