If you have the choice between marking, CC'ing and killing each mob separately or just AoE'ing them down, i think most people will favor the latter.
This isn't even about speedrunning, but the difference between being able to AoE down trash and taking them one by one is huge.
What exactly does people stop from doing dungeons with other people?
You don't have to do it with only your guild, right?
And if you're that desparate to do with your guild, most tank classes will be able to tank heroics even without tank spec, they may take a bit more damage and their tps may not be as good (altough being dps specced may off set this to some extent), but this isn't like Retail where you're missing a bunch of crucial passives / abilities.
It's slower but far from impossible, the gear is probably the bigger factor here.
Yeah, my bad, typed it incorrectly into calc and didn't question the results.