Bad people will be bad in pve gear as well. I agree there.
Concerning a tank who uses DPS gear: I do this all the time for LFG (usually 100% DD) and LFR (partly DD, with a guildie 100%) when I'm sure my healers can handle it. It's all about using the colldowns correctly and keeping the debuffs up what most of the tanks in there won't do. Nothing is as satisfying as achieving 30k dps as a tank and seeing tons of DDs below you