Their iridescence is really great to look at (granted, for early EX, like @wisewailmer said, they really have to be pretty much mint to look really good)
Crowler/Chronos is representative of the elite rigid system, who tries to keep the dead lasts underfoot and especially takes umbrage to Jaden/Judai just casually existing and making a mockery of the elitist system
When you add cards like Tree of Perdition and Unhallowed Phalanx, you start reaping the benefits, including halving each opponents life total each turn
Their first ability is only on enters, and that second ability, while cool, requires setup
This makes everything pop and gives your cards a professional finish
namely, that it couldnt be copied, and, rather than paying life as an additional cost, it should change your life total on resolution