Hhm, well no answer yet on mailing list. Although a lot of times no one answers this kind of question for me.
It just seems like, considering all the things you mention which might be in a group - if nothing else, using them for clipping, and usually they would be duplicated from the existing part of the image, would add unnecessary weight to the file. But it seems like just more stuff that can cause problems, especially if the file gets to be huge.
Or like....what if there was a filter in the group? Filters are known to be trouble-makers, if used unknowingly.
I don't know, it just seems like it shouldn't be possible. But the burning question for me, is there a legitimate way to use a group for clipping.
OOooohhh! I'll bet I just figured it out. It would be legitimate to create a group of objects, styled with gradients or filters for a complex mask. And I guess the ability just carries over to clipping.
I'll bet that's it!