I don't really understand the argument that "you take as much/more damage in the time to cast than you get from the heal."

Whether you're casting cure or not, you're taking that damage anyway. So your choice becomes on between generating enmity and/or doing damage, using cool downs, or healing yourself during that window of time.

I can understand an argument being made that the damage / enmity is more valuable than the heal and I would typically agree. But, me being only level 21, I've been in situations a few times where tossing a cure or two on myself has made me much more comfortable and less concerned with dying.

In a situation where you have no enmity concern, either don't have cool downs available or are saving them for a harder hitting pull, and are taking spike damage that heal becomes the most attractive action. This situation has come up for me personally on a few trash pulls of 3 enemies.