This highly depends on the kind of apps which needs to be done. Its highly difficult to generalize a backend for a n number of apps. I am not comparing the term "backend" here with Parse or YQL or aggregating datasources which is great. You write a backend to custom suite your app so that its highly performant both on the client and server side and you have paas hosting services for the language of choice. I would personally not like to have a generalized backend.