The whole point of it is to help 3rd party developers with the API's, so they homogenize the API's through Play Services. However, you're right about the user

The whole point of it is to help 3rd party developers with the API's, so they homogenize the API's through Play Services. However, you're right about the user part. It will help the users slightly, better apps with latest API's will come along, but the users still won't get the latest features of the OS unless the phone is upgraded to the latest version.