@DeathPaladin While I'd consider someone who dies to mechanics or threat early on to still be bad in general, you are correct in saying that you should "confirm that a bad DPS really is bad DPS. This goes back to Tyler's comment about using meters incorrectly."

I actually had to deal with this very situation a few weeks ago. Our raid leader had changed my role in various fights so that I was helping the group but at a significant loss of DPS to me. I'm a shadow priest, and for weeks I'd been at the top or in the top 5 on their meters. Then on Rhyolith I was asked to dps the boss instead of adds. Of course my dps tanked for obvious reasons. I wasn't being sent up for Alysrazor, which of course put me further down from those who were. I was soaking on Beth Heroic, which gave me little time to dps at all (and the raid leader even stated to me that he didn't expect me to). On Shannox I was asked to do Smite dps to help fill in the gaps for the healers when we were short. That was when a couple of the dps decided it was time to call me out on my low dps-- in raid chat.

So I ignored the RL's assignments for me, followed the strats to give me the highest dps, blew the third spot on the meters and beat them both. Of course, the boss didn't go down, but meters was what was important to them, after all. (Fortunately for me, the RL knew what was up and was well aware that I was about to wipe the raid to prove a point.)

So yeah... Consider the bigger picture when asking whether or not someone is doing acceptable dps.