If we can get producers to standardize all or part of their API, then we gain the ability to switch easily between providers and force them to compete on service instead of proprietary lock-in.
Ideally.
That said, there certainly will be extensions, because REST as it is is not sufficient for many scenarios. Take WebDAV, for instance: it has locks, batch updates, queries, standard support for sync, etc. REST has nothing of this sort. Have you ever tried to sync a large dataset against a typical REST API? It's very painful. While REST is simple, this simplicity mostly comes from being very basic. And WSDL and XML Schema are complex not because their authors are stupid (addition: or because there was some political wars), but because the subject itself is complex.
We have seen this cycle before with WSDL and XML Schema, etc. I hope we have learned from our mistakes and don't go down the same path to have history repeat itself.
Personally I like the idea of an API Specification but the practical use has been a let down so far.