> Would you please tell me which functions or features of vSphere 5.1 can't work without DNS service ? (exclude AD integration)
> or which functions will work better with DNS service, not only for easy recognize than IP address by human beings.
For once your support contract, SSO needs DNS to be in a supported environment. I wouldn't risk to lose precious time just to set up proper DNS resolution in a critical production down case to get in a supported environment first.
In an extremely simple setup everything might work as you expect it to work just on IP basis. Some advanced solutions like VDP and vSphere Replication will not install correctly when you use an IP only network without DNS resolution.
Custom SSL certificates are also a point where name resolution is preferred.