Can a Project Manager not be a Subject Matter Expert?

Can a Project Manager NOT understand the underlying 'subject' that she/he is managing? If you ask a typical PM looking for a job or looking for a promotion, etc. the answer is yes...because the PM manages the project not the actual work.....reality is something different though. As hard as some people try to justify a PM's ability to be isolated from knowing the underlying business (very important) or technology - the lack of that understanding adds risk to the project which if looked at would most likely out way the benefit of the PM's role itself. PM_Value = (Reduction_of_Project_Risk *100) + Increased_Project_Effectiveness + Increased_Project_Efficiencies...or something like that. So, if you're a PM that is working in a business you have no understanding of and in a technical environment that you have no grasp of....how would you be able to know what the risks are? which risks are most likely to occur with the highest impact??? Ask people? If you don't know the subject how would you know which people to ask? by asking other people? people who themselves might not know? Doesn't this lead to increased risks?

I've heard over and over that a PM should be measured by how successful the project was managed not how successful the actual project was.......gee......try to explain that one to a high level exec who just lost millions on a project but has a well run project and nice project plan...