Well, the trick is to use the Plague Marines to cordone off a large swathe of the board, which funnels people into a location where Blightlords can corner them. In ITC, it isn't always about simply tabling someone, and Blightlords are primarily a way to nearly guarantee removal of 1-2 chaff units per turn with combined firepower from other Plague Marines in concert, while largely remaining focused (as an army) on board control overall. Princes are bullies and "traders" -- they jump out, kill something, and usually die afterwards. Marines, however, often benefit from cover, and can deal damage at a very comfortable range while on the move. CC happens, for this army, around Turn 3 or 4, after I have presumably dumped around 400-600 shots into the enemy. Now that we have double the number of attacks on Plague Marines in combat, and considering Arch-Contaminator basically means every Marine is equipped with a Plaguesword, they actually can do work on tougher targets, AND chaff stragglers.
I have more playtesting coming up on Saturday, so I'll let you guys all know how that goes.