I can tell you that in NJ the low voltage smokes/heat/CO detectors with the security system are considered secondary. 120 volt, interconnected, w/battery BU are required, and have been for a long,long time.
Reasoning by state is about exactly what yoy describe; lack of monitoring/service contract + no smokes.
Personally, the secondary LV smokes are a real good item to have (monitored) to notify the fire dept.
