As you have probably guessed this was not done by accident! That being said, I completely understand your dilemma; how to mock the root object? The problem is that in any public API certain things should be exposed and available and other things are better left abstracted/encapsulated from the user of the API. Early on I made this internal, but I think your right it might be a better as a public interface given your feedback.
Can you create a Jira ticket for this feature? http://couchbase.com/issues/ncbc
Note the test project (Couchbase.Tests) is a friend assembly and the ICouchbaseCluster (another note: expect a name change to ICluster before GA) interface can be mocked from there (a weak attempt at soliciting a contribution to the project ;)).
Thanks,
Jeff