Thin provisioning can save you a LOTs of SAN space. You know that storage space on a SAN its expensive. Insdead of thinking how to get enough money to buy larger SAN or upgrade, why not instead migrate your VMs using a VMware Converter with thin provisionned disks?
Here is a little picture if you still do not understant what thin provisioned disk looks like. On the image you can see that the size of the VM on the left is using 10 Gigs on the SAN(but when you look at your VM the disk size inside – it shows that the system has 20 gigs.) It's a magic…. -:)