The Archon w/ PGL is there to support Incubi, really. Incubi don't come with grenades, and lack the option, but a PGL grants both assault and defensive grenades to the entire squad. The Archon is better than the prince for this because both he and the Incubi can ride in the same transport; allies can't ride in the main detachment's transports, and vice versa. The Prince definitely has a lot of use as the head of an allied detachment, though. Guaranteed night-fighting? Hell yes.
"Let the vermin burn. We shall drink deep of their screams this night." Archon Farath Müre, High Archon of the Kabal of the Consuming Flame
(11/5/2013: A few things have been added to the Kabal's fluff, most notably shifting their base of operations into the Webway, sort of, and adding an alliance with the Alpha Legion, or at least a few heads of the hydra.)