Quote - 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?

I think I can answer your question.

A couple years ago I sat down to learn Poser 6 lights and read every tutorial and discussion on the subject. I do remember seeing in more than one thread that IBL lights do not cast shadows.

And in this tutorial, post #34, Olivier says that IBL lights do not cast shadows nicely and to not use shadows with them and instead use a key light to obtain shadows.

http://www.runtimedna.com/mod/forum/messages.php?ShowMessage=184683

I'm not great at making lights, but I do try. I never did get the hang of IBL lights. Maybe it's my technique, but I find that they tend  to make things look flat and over exposed. So I've pretty much given up on using them unless I see they are in a pre-existing light set that I am using. In which case they are usually mixed with a variety of other lights.  When I try making a light set, in order for me to obtain one that I'm happy with, I have to use between 3 and 5 Infinite lights and sometimes some point lights or a spot light. 

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi