Single payor=monopoly. If we did this, we'd be at the mercy of policy changes like reduced reimbursement due to arbitrary budget formulas or restrictions on balance billing (we should never forget these wonderful "gifts" from medicare)
There would have to be strong checks/balances in the payment power of any single payor to make that work. A start would be (1) allowing balance billing and (2) establishing realistic market based rates by CPT codes. The only way I could see to do that would be for the single payor to require each physician to post their menu of charges by CPT in some central place. That way, the single payor could have market based rates that vary by region or city. Patients should also be able to view these rates to help make their primary care decisions.
I agree with gb that the profit motive of insurance companies works against good medical care to patients and realistic compensation to physicians. A return to non-profit insurance companies only might be the start to getting rid of this massive diversion of health care dollars. Single payor could be next step after this but only with the proper safeguards.