I can appreciate both sides of the argument.

It's not nice failing an instance because a teammate is performing badly.
It's not nice being told that you are performing badly (regardless of whether the intention is just to insult you or to try to get you to shape up).

Personally, if a dedicated damage dealer does less damage than my Tank does whilst in my +enmity stance; I'd assume that they are TRYING to be bad.
It's happened on more than a few occasions; usually in PUGs, but there is at least one member of my Free Company that this applies to (no names!)

I'd usually not call someone out over it unless they're actually causing the team to struggle, but I'd certainly not have a particularly high opinion of them as a player.