Here is how I think RMT would exploit it.

You and your buddies are about to fight a NM. You get the claim. You've been looking forward to this for the past week. You've discussed tactics over and over, you're ready for a tough fight...

RMT wants to farm and they want you to hurry up and get it over with. They jump in the fight and are overlevelled/overnumbered for it so it all ends extremely quickly.

Or instead of RMT, it could just be other impatient players who want to hurry it all. Same difference. They'd just jump in to rush you through the fight.

In a more crowded game world, wouldn't most NM fights turn into a overcrowded beat down like behests were originally like?

You still get the gear, but what you don't get is any enjoyment there was to be had from the game play.