This is indeed an interesting remark/question.
I don't know how well your understanding of TG's GI-system is and especially the GI blur radius setting, so if you don't mind I'll explain/show how to...shortly:
Start new scene, set camera near ground and add a sphere close to the camera and let it sit on the ground. Set lighting angle 90 degrees from left/right so you'll have a nice long shadow. Set renderquality to medium (0.6 - 0.7) and GI 2/2.
Render with blur-radius 0 and blur-radius 10.
What you'll see is that with radius 0 bright sharp-edged but smooth shapes will appear in the shadow.
With radius 10 the same detail is just 'blurred' (oh really? lol
) and therefore less visible.
That's the blur radius in a nutshell.
So when do you use what?
Unsurprisingly, this all depends on your type of scene.
In general virtually all images improve from GI 2/2 over GI 1/1. GI 3/3 takes indeed considerable more time to render. GI 3/3 or 4/4 or maybe even higher is something you can better use when features of the terrain are extremely displaced for example. It is then difficult for the render to sample all the displacements properly and this can result in black areas and other lighting artefacts. Increasing GI settings increases the "coverage" of the GI so that light and shadows will be calculated "correctly" in difficult areas.
The blur-radius then determins, well I think, how well GI details will be visible. Like I tried to explain above. So I think you can approach them seperately. However, like you said, if you choose to render with lower GI settings, when you're rendering grasses for example which render almost always fine with GI 2/2, you can reduce the blur radius to "exaggerate" the GI effect.
This is a bit of a philosophical approach of course and maybe here and there not really accurate technically.
Martin
