The ability for people to grow a handful of plants is probably mostly a symbolic one.  It will allow the govt to put the current personal use grow ops that exist through a legal loophole out of business.  These sites dont comply with proper security and oversight and are a public safety issue.  The Federal Govt lawyers will be able to say people who have prescriptions for medical marijuana will have more avenues to buy it and close the loophole.  It will also theoretically not be a big enough target for criminals to steal their stuff as opposed to the larger crops that are getting hit now.