I thought that as well for a while as a Java developer working primarily with UNIX and Windows while diving into the early Mac OS X DP releases. Bits and pieces of that thinking have been challenged for me as I've dug deeper and deeper into OS X and the APIs and how they go together. For example, why would the Finder be Carbon if Carbon was just an interim API? It was Monday's Frameworks talks at WWDC that really turned the lightbulb on as to how they go together.
As far as taking advantage of all of OS X fundamental strengths, such as MP and preemptive threading, Carbon can use the system as well as Cocoa. Where Cocoa really shines is in all of the functionality that you get for free (such as spell checking), the ability to interface with other applications via Services, and a whole host of other really neat things enabled by the object-oriented nature of the framework.