The reasons that this enraged me are:
If the group is clearing the content, the DPS meter is completely irrelevant for a 5-man. DPS meters (especially ones with parsing that can tell you more than just raw numbers) are useful for raiding and can even be invaluable in determining what’s going on in a boss attempt. Using them to mock and belittle strangers you just met in a 5-man is not their intended use, nor is it a good use of them. Share it with your friends — use it to feel quietly smug about how awesome you are all you like — but don’t bring it up in party. If people want to see how amazing your DPS is, they can consult their own meters.
Proper execution of basic mechanics is more important in a 5-man. Frankly, I’m OK if the arms warrior is only putting out 12k DPS if he doesn’t break CC with Bladestorm, doesn’t stand in the black ooze from Sylvanas and die, and hits interrupts to help out on Tyrande. Does the hunter know how to trap the caster dragonkin in the packs before Murozond, and can he use Camo properly to go hit all of the staff fragments in Azure? That guy’s a keeper, even if his DPS is low. Gear can improve his DPS, but it can’t teach a mage how to run over a Flarecore.
It makes the specific times when DPS matters harder to emphasize. If the group can’t kill Murozond because everyone is doing lower DPS than is necessary (and that would mean really poor use of the Hourglass), then I need to be able to tell them this. If you’re running off your mouth all PUG long about how awful Joe the Mage is, it dilutes the effect. I had a End Time run where the DPS players were not capable of killing Muro, and so I arranged with the DPS DK to switch roles with him — and we killed him. I wasn’t rude or condescending, and he wasn’t upset or defensive, as we both realized his DPS was low because he needed the gear the instance has to offer. It wasn’t an ego issue.