And if a DPS has to stop DPSing and not do his full potential damage because a PLD can't hold aggro on a single target boss, how is that called?

Been there lots of times. On the same boss, some PLD can perfectly keep my threat indicator green or max. yellow for the whole fight while I unleash hell on the boss, while with others I'm struggling to not get aggro on me, so I need to constantly stop DPSing.

Don't always blame the DPS.