They would make perfect sense on antennas that use a bowtie style driven element. On the 91XG, which has a modified folded dipole stlyle driven element with the X directors, I found they seem to only smooth out the gain curve on channels above 60.Several different commercially made yagi style antennas use "x" style director elements.
Other types of directors in use are diamond shaped ones and circular ones. I havent investigated their effects. Some shapes look like they were chosen just for looks, heh.