The only situation in which I'd accept a parser in game was if it worked like the duty recorder. All people must agree to it beforehand and it's only there for "high-end" duties where that sort of above-average gameplay is really needed.

In regular content, it's already easy enough to tell whether a single person or a group aren't pulling their weight. We don't need to attach a number to it. If someone is honestly trying to improve their ability to play the game, there are already plenty of posts with advice and guides. There are plenty of theorycrafters who've worked out rotations and melds for equipment.

And as with any feature, we know people would find a way to abuse it, as with any feature. I find the cons of parsing to be greater than their supposed benefit.