B737NG Captain is reluctant to accept a deferred fuel filter differential pressure warning system because its link to the master caution system could not be deactivated. This inability raised the possibility of a false warning during a critical phase of flight.

Narrative

On previous leg; Master Caution/Fuel/Filter Bypass lights illuminated. Completed QRH procedure (which simply notes that your engine may quit); completed leg; landed without incident and wrote up discrepancy. As I understand it; maintenance inspected the filter; replaced it with a new one; and MEL'ed the fuel filter differential pressure warning systems. MEL item 73-4-2 is vague and ambiguous at best; and provides no guidance whatsoever as to what the crew may expect to see when flying the airplane in this condition. I requested additional information as to condition of the airplane; and specifically asked whether the system had been deactivated. Was told that it was still active; and that the warning we had received earlier would probably occur again; and that having changed the filter; we can fly the airplane for another 10 days in this condition. I asked what we were supposed to do when the master caution illuminated and was initially told that we should ignore it because it's a 'known condition.' I pointed out that we could not ignore a master caution; and that if it illuminated during takeoff; we would abort. After some discussion; it was agreed that we should abort; although subsequent conversation with other personnel suggests some confusion over this issue; with some apparently having the impression that whether or not to abort the takeoff for the master caution was a judgment call; since there was a known condition. We were also told that if we did abort for a master caution associated with a recurrence of this fault; we would not need to return to the gate; but should simply get a new release and proceed. We followed; to the best of our ability; the MEL as it was explained to us by maintenance; and followed the procedures as directed by Operations. We carefully briefed the condition and our reject decision before takeoff. The fault condition did not repeat during our next two flights. However; I would submit the following observations. First; there should be a way to deactivate this warning system so that the (presumably) false warning does not occur at all. Second; the MEL item needs to be clarified to provide better guidance for both maintenance personnel and flight crews. Flight crews need information as to what they are likely to see; and what actions they should take when flying an airplane with a warning system that produces this intermittent fault condition. Finally; there should never be a known condition that results in a master caution that crews are supposed to ignore.