When you delete a post in a set, its parent automatically takes its place in the set (as long as the appropriate checkbox is checked).

What about posts in pools, though?

I would say no it doesn't, usually because of edits

Yeah although it does sound like it'd be nice, the issue is edits, as RD said. For instance, if there's two pools, one of which is a comic and the other is an English translation of the comc, and the English posts are children of the original posts, then you wouldn't want the posts in the English-version pool being replaced by the non-English versions if the English versions were deleted.

There might be other examples, but that's what I have off the top of my head. I don't think this is an issue that has to be addressed very often anyways?