IBL + sphere, IDL OFF- sphere does not block the light and provides no light itself, but does show in reflections.

IBL + sphere, IDL ON - sphere blocks the IBL and provides the light and shows in reflections - the IBL does nothing

Note that the only difference in these two scenes is enable/disable IDL in render settings.

Does this mean you should remove the IBL?

Not if you're doing test renders. When I want fast test renders, I can turn off the IDL the IBL will take over providing some world-light. Flip IDL on, and the IBL is ignored.

Also (and I don't ever bother doing this) if you make the sphere even partially transparent, then IBL light will partially pass through.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)