Limiting the game to use just 1 OnlineSubsystem is very limiting. I would like to use facebook and google play services for example. AFAIK there is no out of the box way of using more than one online subsystem right now.
I just want to make sure I understand what you are asking for. Are you talking about 3rd party advertising systems or are you referring specifically to structures such as the android store, facebook apps/games, itunes, etc? For these in particular they do require that you package the project and submit them to the individual locations.
In OnlineSubsystem code (c++) I can see several online subsystems which each one implement a subset of IOnlineSubsystem interfaces (IOnlineSubsystem).
I can see Google Play Online Subsystem, Facebook Online Subsystem, IOS Online Subsystem (which I guess is ‘GameCenter’) etc.
But you can use just one in your game (as far as I know). So it would be desirable to be able to use google play services and post results to your friends facebook’s wall or whatever. I mean, why not let the programmer to use several online subsystems instead of limiting to the use of only one?