edit: Looked up answer myself. Lein downloads whatever key the signature claims to be made with from public keyservers. How does this provide any additional security over not bothering to verify signatures?
The difference is that you could track down the keys either directly from the author or by someone who has already personally verified and signed the author's key. In practice this is very difficult, and using a key that you haven't gotten your friends and co-workers to sign is not any better than skipping the signing altogether.
Even if you have carefully installed the correct key from the author, if your download is intercepted and an attacker sends you a bogus artifact and signature it looks like Lein will just retrieve the attackers key from the keyserver and validate the signature.
This is true; at the time of implementation so few Clojure libraries were signed that taking it the rest of the way was not a clear win.
But clearly the job isn't finished; even if Clojure developers do a good job of signing packages and signing each others keys, (which is not generally true today) it still needs to distinguish between signed packages and trusted packages. Hopefully the next version can add this. But as with anything that requires extra steps from the developer community, a thorough solution is going to take time.
edit: Looked up answer myself. Lein downloads whatever key the signature claims to be made with from public keyservers. How does this provide any additional security over not bothering to verify signatures?