The Boolean function is a special case in fuzzy logic, and it essentially mimics the way most data classification algorithms work: it either 'IS' or it 'ISN'T.' The Boolean function does have uses, however. In Fuzzy Logic one can make the membership functions overlap as  needed. A Boolean function could certainly be set  up as a 'watchdog' that could be used to force a decision should the decision algorithm have a flaw. For instance, let's say that your data is speed in MPH, from 1 - 100. You decide that  0-15 is SLOW and taper it to 25mph using a  trapezoidal function. The MEDIUM SLOW set may start at 15mph and ramp up to another trapezoidal function's linear area at 35mph. The  Boolean MEDIUM_WATCH would take effect at  15.1 mph to say, 55 mph. Now let's assume that for whatever reason that a data point is read  that is 50% SLOW membership (20 mph) and ALSO 50% MEDIUM membership. Your decision algorithm, depending on it's robustness, could miss this data. The Boolean function, however, acts as a watchdog such that in the event of an equality of  overlapping function membership, the Boolean function can be used to make the decision. It would not normally be consulted, but when it is, it would work well.
