I use high GI sample quality for close-ups with very dense vegetation with mixed properties like grass, flowers, etc. I've found that it doesn't have as high impact on render times like GI relative detail - that's why I'm almost always using it at 1.
I do agree that for most cases more than 1/3 is unnecessary. Especially if doing large renders (3000 x 2000px for example).
Probably we need to make a wiki entry with different scenes and different render settings (especially GI) to make visible comparable results for everyone.