Hi Nikesh,
CMIS defines multiple bindings to increase the adoption. When you design a new interface or standard today, you probably would go for a RESTish interface. And that's why there is the CMIS AtomPub binding. But, it is a fact that many companies spent a ton of money a few years ago into a Web Services infrastructure. That includes security services, identity services, auditing services, and so on. The effort to incorporate a REST interface into this environment would be huge. But, if CMIS would only define a Web Services interfaces, most modern systems would not touch it - for technical and ideological reasons.
So CMIS leaves the CMIS client the choice of what fits better into the IT infrastructure. And on top of that, Apache Chemistry hides all those low-level details.
If you, for example, implement against Apache Chemistry OpenCMIS, you implement against a
Java interface. Which binding is finally used in the production environment (almost) doesn't matter.
Regards,
Florian