Insurance coverage for medical procedures

I have some questions about insurance coverage that I thought I'd throw into the blogosphere.
Who determines whether a certain medical procedure that reduces risk will be covered financially by the insurance company?

My question comes from issues surrounding prophylactic mastectomies (removing a breast even though it does not have cancer). Given the improvements in cosmetic outcomes of breast reconstrcution, many more women who have breast cancer in one breast are interested in having the opposite breast removed as well. In general, there is about a 10% chance of developing breast cancer in the opposite breast when you are diagnosed with breast cancer. Is this procedure paid for under this particular situation?

Another example: a 30 year old woman with a family history of breast cancer, her mother had it, her older sister had it, has recently tested positive for the BRCA1 gene, meaning that she has about an 85% chance of developing breast cancer in her life time. This woman wants bilateral prophylactic mastectomies. Will this be covered by insurance?

The core of my uncertainty with these two situations is that in neither case does the medical procedure "treat" a disease, it only reduces risk. My question is, at what risk reduction is this procedure considered appropriate? What if the risk of developing breast cancer is 5%? 15? 50? 90? And who has the authority to deem the "medical appropriateness" of this procedure? Let's say that one doctor deems that a particular woman has a 20% risk of breast cancer and this risk warrants a prophylactic mastectomy. What about financial coverage of this procedure? Will insurance companies listen to this physician's logic? Do insurance companies have their own formulas for what risks they are willing to take on (ie deny coverage) compared to what is "too risky" (ie they will pay to reduce the risk)?

In terms of what I think, I think that if a doctor has evidence in the medical literature to reduce risk by doing the procedure, and if the patient is highly risk averse, then the procedure should be covered--assuming that the risks of the procedure are minor compared to the risk potentially reduced.

This is also a situation where the grayness between evidence-based medicine and consumer-based medicine come into play. Let's say the doctor does not feel that the procedure would be a good decision, but the patient really wants the surgery. She doesn't care if the medical literature states that the mastectomy would minimally reduce her risk. Psychologically, she "wants the breast off", "doesn't want to lay awake in bed every night worry about it" etc. What's the role of the physician in this situation? To be consumer-based, to do the surgery because of the patient's desire. Or to be evidence-based and not perform the procedure? How does costs play into this decision? What would most doctors do in this situation? What ought doctors do? What would insurance companies do in this situation What ought insurance companies do?