death's dance should make you take more damage than you got hit with

i.e delays 30% of damage you take but ups it to 40% over time (so you're taking 110% damage total, but 40% over the next couple seconds)

it's good that this item is actually bought on more than one champion but when it delays so much damage you're going to easily be able to heal it back up, which is why it's so strong on a big chunk of ADs and a couple of ADCs .

therefore you should take more damage over time than you receive, to balance out the ability to drain tank through it