Jerry McDonald of the Inside the Raiders breaks down the decision from all sides. The offense moved far better under McCown, although it made more errors. McDonald notes that Culpepper can't run:

It's clear at this point the Culpepper is not the same guy who Warren Sapp called the "white rhino" when Culpepper was at Minnesota. (The white rhino designation had to do with Culpepper's size and power for his position and the fact that white rhinos are extremely rare). ...

Culpepper has been an extremely popular teammate and a classy guy from the day he arrived, acting without any sense of entitlement or expectations. But on the field, he is not likely to be the old Daunte any time soon, and certainly not by the time his one-year contract expires with the Raiders.

My gut instinct says that McCown will be back under center when he's ready, no matter how unpopular a decision it is. Kiffin thought McCown knew the offense better all preseason, and that's still probably true.