Most "nukings" can be undone. However, the "gain" is a record of why the node was deleted so that a determination can be made as to whether or not it should be undone. It also addresses issues like someone bookmarking a question and coming back to find that it disappeared. A reaping would likely leave a pointer to a better source of similar information, for example.
Also, a reaped node doesn't require more space than a nuked node. And the percentage is so incredibly tiny that it doesn't matter anyway.
I think the idea is fine. I doubt it will be done, however. Someone would need to implement filtering of reaped nodes from all of the several CatQ+A displays and then implement the reaping mechanism.