Same with the people app (though /some/ of that is promised).
Unfortunately, companies being companies, they only cooperate sparingly and over all, complete interaction between all major internet "cloud" service providers it not something that's going to happen in any large way.
But your right, My cloud storage app, email app, people app, calendar app etc should all be cross platform and cross service. Picking the best one in each location but access to the sum total of all my data no matter where it's stored.
Usually you need to look at third party developers for that (i.e. trillian etc) but even then service and features are always spotty/incomplete , largely due to the service providers needing you to be using /thier/ clients which support /their/ ads.