I imagine there are some renowned physicians in Europe and Asia who also don't know that Toronto is a Canadian city. Or other arbitrary pieces of trivia for that matter. It doesn't stop them from being successful at their job.

Conversely, knowing which city has airports named after a WWII lieutenant and a WWII battle doesn't protect from malpractice.

At the moment, it's just diagnosing. Hopefully a) watson will provide the reasoning, enabling debugging and obvious errors to be spotted, and b) the doctors will continue to filter watson's answers for some time, shielding it from liability.

They went through a round of liability when expert systems were medical introduced in the 1980s. I dont know if they ever past the toy stage,

Expert system-like rule-checking is imbedded in pharmacy programs and the such. but not advertised as the central engine of the code,but an auxilary.

I imagine the same as CT/X-Ray manufacturers. It's going to be used as a tool, not a replacement for an oncologist.