IBL doesn't do specular highlights, but it does do shadows.  Maybe they're mixing up some thing they read...

This animation has IBL shadows as well as an infinite Sunlight casting shadows and spec highlights.

http://www.veoh.com/videos/v6562766RwCWEZhS?c=3dArt

Quote - > Quote - You need at least one extra light, either a spot or an infinite, since IBL/HDRI lights don't do shadows on their own. That's my two cents worth at the moment. (whistling and waiting for Acadia the link goddess to make an appearance)

Why did you say that?

I just got done saying that you can use shadows on IBL, and what to think about when you do.

Not trying to beat you up, just curious. I have seen more than 5 times people claiming that image based lights cannot have shadows on them. Where does this strange idea keep coming from?

Have you every tried it? This is a puzzle to me. That people keep saying absolutely incorrect things that can be disproven by a simple 20 second experiment.

Why does it keep happening?

I've written at least 5 interesting how-to threads about shadows with IBL. Am I invisible again? :)