Limiting the percentage of non-owner-occupied units is, in my opinion, the best way to approach this. In my community, everyone who moves in is issued a key card for the pool and recreation buildings, a remote control for the gates, and must provide a telephone number for the intercom system. We know who the renters are, because they have to apply for access. Some CIDs require valid parking stickers, as well, so that's another way to monitor this.
Here's a good article on the Davis-Stirling site about this subject.
Restricting the percentage of rented units is necessary for insurance purposes (typically 30-35%) and for FHA certification (no more than 50%). Grandfathering should be the last resort, IMHO. If the percentage of renters is not controlled, one day homeowners could find themselves owning an apartment in a run-down apartment complex, instead of a community of homeowners. It wouldn't be the first time this has happened.
Rob