It gets a little ridiculous at times.

Come across a Wyrm. It casts darkness.
I cast light. I take a swing.
It casts darkness.
I cast light.
It casts darkness.
I cast light.
It casts darkness.
I cast light. I take a swing.
Repeat...

I mean, really, if the darkness is negated, you'd figure it would give up on that spell. It's clearly not going to work.