I haven't put much thoughts into non 2/2/4 comps since 3.x contents were mostly designed around having two tanks and/or two healers. Even susano and lakshmi seemed to follow that trend as well. Making pld a "solo tank" job would also make its dps lower than other tanks, thus punishing people when they queue into 8 man contents with a pld. Giving pld stances for super tanky mode and normal tank mode and dps mode would just overcomplicate things and make it confusing for new plds.