I have used them. Mostly I used them for debugging, rewriting, "code sketching" as it were. Without them, extending or reextending a function at the REPL is just annoying; if you ever make a mistake in the test function you have to reload the original function. I suppose it might make sense to have two macros - extend and extendl, maybe - one which doesn't take a label, and one which does.
Undo & reextend could easily be implemented on top of labeled extend - just store the label last used when extending a given function in a table, along with whatever information is needed to undo that extension, and have an unlabeled extension macro that uses a gensym as the label. That way we get labeled & unlabeled extend with undo & replacement.
