Root's antlion point was similar to his purple nymph point: Picking a creature and then only allowing the pillar/pends of that element for the creature+that creature in duels is bad, but obviously there would be counter measures against that.
There's also the fact that unless there's a somewhat strict guidelines, duel rules would have to be pre-approved before doing (otherwise wasting both party's time with an invalid match) which makes this event longer than it needs to be.
But I like the unlocking idea.