Duct detectors are needed if the unit CFM rating is 2000 or greater on the return side. If your unit is 4000 or greater you need them on supply and return. About the only thing they do is tell you that your duct has dust in it when the heat comes on.As far as early fire detection they are useless. Think about it you have a small fire in a room your HVAC is drawing clean air from other parts of the building when it gets back to the unit the air is so doluted there is no smoke to detect. But I know THE CODE REQUIRES THEM. I use to work for a fire alarn distributior we had Notifier and Pyrotronics equipment.
I would like to add that if the total CFM of all units in a discrete area (say a single tenant) is 2000 or more then the detectors are required. Not just the CFM of any one unit. They must be wired for unit shutdown and be monitored by the buildings central system. I'll quote NFPA when I find my book in this messy office.