Long term cordial neighbors definitely serve each others interests, even if there is no formal alliance. All I know is that EA Pan is comfortable aggregating the majority of his forces along his northern border, leaving the vast southern border that he shares with the undead unprotected. In the grand scheme of things, he may not be a direct ally (coordinating attacks on a mutual opponent), but he is definitely a facilitator. And by choosing to facilitate Ermor, Pan goes down as Evil in my book.
Although, now that sum1one reminds me that Pan's troops have recuperation, I am not sure remote disease spells will have much effect. I guess, I will have to just wait for them to come out of the castle so that I can kill them off in hand-to-hand. 150 vs. 2000. Gotta love them odds.