This is actually a good one.

Presumably the sublicense can't grant more rights than the previous licensee received from the original licensor. Also, if unmodified source was passed on to the sublicensee, I'm not sure how the sublicensor could enforce that they've passed on fewer rights, since it would be up to the copyright holder to enforce.

The sublicensing ability only really seems useful if you're producing derivative works (where you also hold copyright on some parts), or when you're distributing binaries (where it would be difficult to exercise other rights).