When to Separate vCenter and Single Sign on Server

With 5.1, I have the option to have the Single Sign on Server separate from vCenter Server.  When is this necessary and appropriate and when is it overkill?  If I am using vCOPS, VCD, and other components?  What are the factors that influence the decision to need to separate them such as:

-having many components in the environment needing authentication

-Having a heavly load of authentication traffic

-Other factors?

Thanks for your input.