In my case, i think the problem is that the neutral wire is shared
between the two alternating circuits. So if the continuity of the
neutral circuit is interrupted because of a bad outlet or something,
pigtailing will avoid that problem. Anyway, that's my take. Both my
Black & Decker DIY books show pigtailing and thats the recommendation I
see in these groups so that's with I'M gonna do. :-)

Kevin