I was at a Dell EqualLogic training event and the speaker threw out the idea that it may not be a bad idea to create a single datastore to house a single VM rather than making a large datastore and putting several VMs on it. His reasoning was that a 1:1 ratio of VMs to datastores was a better option for managing snapshots created by Dell's Host Integration Toolkit Auto Snapshot Manager. He also threw out the qualifier that this is most beneficial for high-value VMs and not necessarily every VM you have.
The only reason I'm aware of why you wouldn't want to do this is because each datastore that vCenter has to scan adds to the total amount of time it takes to scan your SAN.