Dan, that's a bit my point. The Z letter tells you to consider an immediate landing. However, with the possibility of a false alarm, ditching in the middle of the ocean might be as deadly. I'm not even worried about hull loss, but life loss at ditching in unprepared surface. I say it's a matter of risk management, and as for now with the resources we have, ditching might still be better, but it would be nice to have some sort of visual confirmation of actual fire (such as cameras in the cargo hold or something).
Examples of false fire alarms:
Incident: Emirates A332 near Colombo on Nov 2nd 2013, cargo fire indication
Incident: Delta B752 near Pasco on Oct 21st 2013, cargo fire indication