Evidence does suggest fairly strongly that the people who use medical marijuana for cancer or AIDS is a small percentage of all users.
Links
[1] http://www.politifact.com/florida/statements/2014/may/29/dont-let-florida-go-pot/less-5-percent-registered-users-states-allowing-me/
[2] http://rethinkpot.org/tags/amendment
[3] http://rethinkpot.org/tags/cancer
[4] http://rethinkpot.org/tags/aids