While it's definitely tempting to add such systems, especially if you're knee-deep into the endgame aspects... parsers and other performance-related tools do represent a slippery slope for player behaviours.

Just from my own experience playing WoW, it's evident that these led to the game pretty much becoming dominated by a performance-obsessed mindset that put optimization and efficiency above all else -- including actually enjoying your time playing the game. To make matters even worse, it bled into ALL levels of content; new players were not spared from the demands for optimization and efficiency, basically making that game unapproachable.

The current "detente" between FFXIV and parsers where a blind eye is turned to them if they aren't causing a problem despite being against the rules is probably the best situation that I can think of. Making it more widespread might make it easier for those who are concerned with a good performance, but it will be to the detriment of everyone else; the snowball effect couldn't be stopped.

---

I recall reading/hearing that the developers for FFXIV actually did consider implementing these tools at one point (may have been during one of the Live Letters)... but promptly stopped working on it when they saw it was affecting their own behaviour, namely making them more unpleasant to be around because of the strong competitive aspect they invariably start playing up. And again, from my own experience with WoW, I can actually attest to this. It was pretty much inevitable that I tried out using a damage meter... and I found myself being more and more of a jerk, and I wasn't liking it. I stopped using it pretty quickly after this realization, and I suspect Yoshi-P saw the exact same thing happening when his own team got a hold of them for FFXIV.

Needless to say, it's likely why he's dead-set against implementing them... they affect player behaviours and cause social problems.
Competition isn't always a good thing, especially when you're trying to work together.