No, it's actually a lot simpler than that.
The moment you attach a number and start measuring players, casual players feel under pressure to perform well; it completely drains the fun out of the game for them, period. The only measure they really care about is whether or not the boss goes down, and casuals are usually at least trying to contribute. It's that act of attaching a number relating to their performance just undermines the experience for them; they don't find themselves able to enjoy the game anymore as their either worrying or dreading the results of their performance being measured.
For those who do care about measuring and improving their performance, and even enjoy doing so... that's the point you've left the "casual" mindset and are starting to approach the "hardcore" mentality.
This isn't a stab at the "hardcore" crowd, but you have to recognize that a LOT of players really don't want to even think about measuring their performance; the burden of that will ruin their experience with the game.
There's also the issue of keeping parsing under control. Throw out the claim of being reasonable all you want... it NEVER stays there, often ever-escalating to the point its impossible for any new or learning players to gain a foothold on even content that's intended to be easy. I've seen it happen many times before, unfortunately. Heck, I've even used similar tools in other games and I didn't like how they affected my own behaviour; I caught myself turning into a jerk and obsessing over minor details, so I promptly put an end to it right then and there. Even if players aren't expecting "perfect performance" now... it slowly but surely becomes "exceptional performance in ALL content", including content where it has no place in (including the lowest level dungeons).
And I wish I wasn't speaking from experience here... but that's the truth of the matter. The tools can be fine when they're used appropriately.
The trouble is that unless something like the current detente SE has with them is in place, they are *GUARANTEED* to be abused.