BOS Controller elected not to issue a go around after an obvious false AMASS/ASDE-X activated; the reporter noting that if the false target and resulting false alarms could be fixed the system would be of great benefit.

Narrative

The aircraft in question was in conflict with a ghost target that the AMASS/ADSE-X had generated. I was working Local Control East in the 4/9 configuration. I had control of Runway 4R and 9. Aircraft X was cleared to land 4R. 4R has a displaced threshold; and Aircraft X was just passed M1 Taxiway at maybe 50 FT in the air when the AMASS alerted saying 'go around' or 'Runway 4R go around' I don't remember exactly what it said. The AMASS circled Aircraft X; which was just passed M1 Taxiway; and it also circled a target at the approach end of Runway 4R. Both circled targets said 'Dupe ID'. The field was VFR and I could see Aircraft X out the window and was easily able to correlate its position with the correct target on the ASDE-X; and I knew Aircraft X was already passed the false target that it was in conflict with. I also do not remember if the AMASS/ASDE-X specified which target it wanted me to send around. In my opinion; Aircraft X would/could not go around even if I told him to go around. I did not send Aircraft X around. Our orders do say that if the AMASS tells you to 'Go around'; you must send him. And this is why I'm filing this report. I would do 3 things. 1) I would change the parameters on the system to give the alerts in a more timely manner. In my case; there was no time to send the aircraft around. 2) I would change the procedure saying that we must react to what the AMASS/ASDE-X says. With the amount of false targets generated by the system; this is going to continue to happen. Assuming we get more time to react; I would let the controller evaluate what the system is telling us; and let the controller decide what to do. The most important of the recommendations; and 1 and 2 could be disregarded if: 3) We fix the AMASS/ASDE-X to work correctly and get rid of the false targets which not only set off false alarms; but also distract the controller even when the alarms are not going off. If the AMASS/ASDE-X system worked properly; it would be a major asset to the controllers; as it is now; the most helpful part of the system is the data tags; which is a shame because I can see the tremendous potential of this system.