Waste of time and being combative with passengers with wildly inconsistent enforcement of bags that touch the edges, or were compliant sizes until the airline changed its rules, or are currently compliant with other airlines' allowed sizes, and fit in the overhead anyway in the correct orientation because mainline airplanes have basically two actual bin sizes (and bins in regional planes generally can't fit the allowed carryon dimensions anyway.)

Really, the only stupidly large carryons I see people trying their luck is the occasional 60l+ hiking backpack, which are obvious enough that gate agents could force them to measure.