I'm usually not that much of an API fanboy, but in the case of social networks I really feel that they need one.
If you're active on several social networks then being able to share content to more than one of them at a time is kind of important and being able to do so without having to login to each and everyone of them is useful.
Of course if you cross-post ALL your content between ALL the social networks you are either very stupid or don't post that often, as you'll probably annoy quite a few of the people that are connected to you across more than one network.
So with the launch of Google Plus there's definitely a need for them to provide some form of API - if only to allow people to manage updates etc.,
Though I suspect a lot of what Google is trying to do is to get us all to be permanently logged in .. .. (Now if they fixed the multiple accounts issues that might be feasible .. )