Of course at the beginning of the Rhapsody project, the two APIs had nothing to do with each other. But I have to point out that Cocoa is no longer the cross platform OPENSTEP. It came from OPENSTEP, but has changed along the way. There is no way to take a Cocoa app (in contrast to an OPENSTEP app) and compile it to work on Windows or wherever else. And, in my opinion, if you want truely cross platform code, go for Java.
Now, as to the benefits of OO vs. Proceedural -- I'm a huge propoent of OO. I have been writing nothing but OO apps for the last 6 years or so and wouldn't go back to writing proceedural apps. Theres no question that Cocoa has an advantage over Carbon -- a huge advantage. But, every good OO programmer should know what is underneath the system that they are using. And know that it is not an either-or decision. In fact, to me one of the real strengths of Cocoa is being able to drop down to Carbon if needed. Or all the way down to BSD.