A few hidden services have added an HTTPS cert but I think that's mostly for a publicity stunt than anything else.
As indicated in the roger’s lecture, HTTPS is usefull for HS : - browsers handle more securely cookies or other stuff in HTTPS mode, avoiding some possible leaks - because anybody can create an HS and proxify any content, X.509 certs allow users to verify the authenticity of the HS (you are on the official Facebook HS if you have a cert with facebook.com *AND* facebookcorewwwi.onion inside)