I'd say that it depends on the service. If I'm using an online solution to track my finances, or manage my businesses project timelines, then I need the security of portable data. If the service were to disappear one day, I would need to be able to replace it.
Social sites are a reflection of something that I already own. As I encourage friends to join and build out a network, the online persona starts to look more like the reality. Making that data portable would make it easier for me to jump ship and move to a new service, but nothing would be irreparably lost if I had to start again somewhere new.