Forums are ok for wars to arrange if you have all days players online who can play wars.
But with XFire you just could ask players from other clans if they have players online for a war for the same evening (moment). In the past it wasnt a problem to arrange wars for other dates but those days it is not for all clans possible, because one day 6 players other day 1 or 2 and players dont have time anymore to visit the websites of their clan
So it was easy with XFire to arrange wars for the same evening, because you could see on your friendlist who was online.