I believe that must-revalidate means :

Once the cache expires, refuse to return stale responses to the user even if they say that stale responses are acceptable.

Whereas no-cache implies :

must-revalidate plus the fact the response becomes stale right away.

If a response is cacheable for 10 seconds, then must-revalidate kicks in after 10 seconds, whereas no-cache implies must-revalidate after 0 seconds.

At least, that's my interpretation.