Online backup software "Aggregator" software, does it makes sense[image included]?

A resounding yes! I'd use it.

Interesting idea! But I think it would be overwhelming for the typical end-user. Usually regular users would typically stick with one or two services therefore, not really needing such an aggregation service.

Your main target however could be power internet users and geeks who store different stuff in different places. Some people would do that to store different "categories" of stuff at different places. Additionally some of the above services are already integrated with different softwares/services already thus, some users put their stuff there anyway when they perform daily tasks like social networking or work duties (Dropbox for instance can be used among professional teams). Moreover, storing different stuff at different places will ensure security (i.e harder to track your stuff, interests, areas of work, ..etc) and you could do that for data redundancy and more safety.

Professional class businesses I believe, are outta the scope as well since they already have their contracts or own advanced backup services. They usually have internal backup servers with redundancies (i.e No single point of failure architecture) and already contracts with cloud-based storage services (such as amazon) too. They have incremental, full and differential backups acc. to their needs.

If you really need developers, you could post your idea to the freelance community and see if you could capture the interest of some programmers. An example would be http://gun.io/