I've heard though that A3 is a pain to do on the MNK because something is guaranteed to fall off all the time, or was it some other fight? I can't really remember.
Still, even if that is the case, i.e. encounter mechanics making a specific class always perform at abysmal capacity---not that I'm saying MNKs can't do over 600 on A3, just an example---having an official parser would allow the community to quickly come to that conclusion and provide feedback to the devs.