They will not add DPS metter for the same reason chat is disabled in PvP, even if harrasment is against ToS, people would do it more often when the option is in the game and mods will have extra work.

On other side, FF is not made to have any DPS metter and (surprise, surprise), DPS metters are not accurate at the moment and they were never accurate. Big portion of the DMG is calculated with formulas or simulations to work around missing numbers or information. If SE are releasing thier own metter, they should provide accurate tool that guarantee proper numbers and with the current system that is not possible. I don't believe they will rewrite the entire workflow of dps'ing to help the part of the community that just can't understand how the battle system works and can't live without DPS metter.