With vSphere 5.1 a new component called Single-Sign-On (SSO) has been introduced. The new SSO service is mandatory since 5.1. There is no way around, you have to use it. The good thing about it is that it has various authentication options and can be deployed in an redundant fashion. Unfortunately it adds a lot of complexity to your configuration but if you understand all of it's components and functions, you won't miss it.