Ok, here is the KB article discussing the problem where the CPU usage of a virtual machine might increase significantly after VMotion migrates the virtual machine in a cluster with DRS enabled. As a result, the performance of the virtual machine might be degraded.
“Starting with ESX Server 3.5 and VirtualCenter 2.5, DRS applies a [...]

If you have a DRS enabled cluster in Virtual Infrastructure you might not want certain virtual machines to ever run on the same ESX host.  They might have certain disk I/O or network behaviors you want separated, or it might just be a case of uptime robustness you want like in the case of multiple [...]