I'm no expert, but I can imagine that automatically opening an air vent to the outdoors, when a fire is detected, isn't a great idea.
I did subscribe to a monitored alarm service specifically for the smoke detectors, on the theory that while I have little need for a burglar alarm here, I would really want the fire department sent if something started while I was out. I'm on my local department, and usually home, but the neighbours wouldn't know quickly if something happened while we're away.