According tot he recommendations in the vCenter 5.1 Database Best Practices white paper, VMware recommends the following layout for disks of the SQL backend server:
It is recommended to have 5 disks:
mssql01 - master and msdb databases(.mdf, .ldf)
mssql02 - tempdb(.mdf, .ldf), also set the initial size to 10GB
mssql03 - VCDB(.mdf, .ldf)
mssql04 - VCDB Backup location
If the SQL server is virtualized, wouldnt creating seperate virtual disks be pointless unless all the virtual disks were created on seperate VMFS volumes? I have 2 sites, one we just brought up with infrastruture chosen for a second site for SRM. The second site does not have multiple LUN's currently as at this point there is no requirement...except maybe because of this SQL database segrec=gation.
In addition, it would be a good idea to put it on different virtual scsi controllers too. The idea is that you'd be able to increase vmdk's for specific disks w/o huge overallocation. It also reduces the possibility of the virtual scsi adapters being a bottleneck and if you do decide to move the vmdk's to a datastore cluster, then vmdk's can be moved individually to different datastores based on I/O profile.
Bottom line, they recommend it this way so that you gain flexibility and performance down the road.