Respectfully disagree.
Volunteers can supplement the existing 911 system, but they shouldn't replace it, nor do they reduce the responsibility for the AHJ. Using the Sterling Rescue example, who is responsible for EMS coverage? Sterling or the county? If it's the county, then an ambulance should be budgeted for and staffed to provide proper coverage, regardless of if Sterling has an ambulance available. If Sterling has the ambulance available, they can take calls (provided they notify dispatch that they are in service), and the county ambulance can be relocated or be the backup and enjoy an easy night. If not, then the county can cover the calls, as they are their responsibility. If it's Sterling's responsibility, than they decide how they want to handle coverage (but that really sounds like a bad idea, esp if they have staffing issues)
If a volunteer ambulance wants to "play" in the 911 system, why not let them? Assuming they are closer, answer all calls promptly, and follow the same rules and regulations that other 911 ambulances need to follow, why not? I think having them listen to the scanner and buff calls is both stupid and bad, but if they are dispatched, because they are trained, equipped and closer, then why not? And if they are not in service, than the Authority Having Jurisdiction needs to provide coverage, just like they would if the volunteer agency didn't exist.