The Boolean shape is a special case in fuzzy logic, and it essentially mimics the way most data classification algorithms work: it either 'IS' or it 'AIN'T.' The Boolean shape does have uses, however. In Fuzzy Logic  one can make the decision shapes overlap as  needed. A Boolean set 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 set. The MEDIUM SLOW set may start at 15mph and ramp up to another trapezoidal  decision shape'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 set, however, acts as a watchdog such that in the event of an equality of  overlapping set  membership, the Boolean set can be used to make the decision. It would not normally be consulted, but when it is, it would work well.
