The guy was a major dick and as others have pointed out, it was more than likely to fund his mount instead of repairs. Yes plate & mail cost more to repair in vanilla than cloth did but it wasn't outrageously high. At 60 and full epics the difference between a plate repair and a cloth repair was maybe 20-30% if even that much. When something like that happens again (and it will because the community is far more toxic now than it was back in vanilla) take to the LFG & trade channels to blast him for doing it. People literally blacklist people for doing that kind of shit. Also next time kick him from the group if someone starts pulling that shit again. Yes tanks are needed to run stuff but they aren't irreplaceable. And TBH most dungeons can be ran without at tank before the lvl 55+ ones. A "real" tank just makes things go faster & easier.