ESXi VM's on a SAN, multiple datastores better?

Using a  3PAR StoreServ 7200  for datastores shared by a couple of vSphere ESXi Hosts running a small number of VM's on each host.

From a performance point of view, is it better to have multiple datastores, with each VM on its own datastore rather than putting all the VM's on a single datastore?

 I'm thinking that maybe separate datastores are similar to separate hard drives and you want to divide I/O between seperate datastores to speed things up.  Does this make sense?  Or does it not work that way with datastores on a SAN?

Thanks.