User defined number of shells by feature or by region/area would be useful when threading holes. If a part only needs 2 outer shells but you have threaded features, the part would fail if you print with 2 shells. 2 shells for threaded features is not sufficient. I've tapped 2 shell features before and I end up tearing the entire section clear off. It is a waste to have to print 4-5 shells for something that only needs 2, just to be able to tap some holes.
It's probably best to use heat-installed threaded inserts, but what if you don't have them, or don't want to use them? The option should be there.
