Marking targets is important. If there are 3 mobs and you've marked all of them, it gives the DPS a sense of what to hit next. If you only marked 1 mob out of 3 then after the first one goes down you're going to have trouble with threat unless you've been keeping flashes up.

This is just my opinion, but I think people who are having trouble keeping threat on multiple mobs are the ones who don't mark.

If you've been marking, keeping flashes up and still losing aggro, then it might be an over-zealous DPS or something.