my post was actually about the comment made before stating that it was quite possibly not prohibitively expensive to render the cubemap every frame. I would still argue that in the deferred setup, it is pretty expensive, given the number of passes you need to do to get the final results. (you could of course just forward render with limited lights / sunlight only, but it's tricky to get looking right).
Certainly cheaper than rendering the scene and lighting the scene 6 times.
Like I said, rendering a cube map every frame obviously has some cost versus not doing it at all, but you can pretty much ensure that rendering the whole cubemap happens within some (small) factor of the time it takes to do the main render pass by rendering at a reduced resolution, so I don't think it's really prohibitive at all.