Last night I and a couple of guildmates signed up for a frost run and we ended up with two DPS and popped into Azjol-Nerub. I'd got myself a new fist weapon on my shammy and wanted to punch things in the face. Half way through the run, the healer sent me a tell asking me to inspect the DPS warrior we'd picked up. And yes, he was DPSing in Prot, with all of his talent points in the protection tree, in a mix of tanking and PvP gear. Yes, his DPS was low (I believe the proper phrase would be 'ridiculously low') and yes, he ran and pulled Hadronox instead of letting the person signed up to tank do it, and brought all the adds over as well. I was all set to blow a gasket at him, when I realized that this was a PuG. I'd never see the guy again. I was putting out enough DPS to cover for him, as was the hunter in the group. We burned down Hadronox, I briefly explained to the guy what the adds did, he apologized as this was all still very new to him and it was his first 80.
This led to a brief discussion while we were heading down to kill Anub'arak.