They served Balor because they knew he'd destroy all extant states, and they hoped to at least create an empire of their own;then they refused Soulblighter and were persuaded to ally with Alric because they saw in him a champion of civilization who had a chance of winning.
I beleive trows served balor because they freed him from their prison underneath the cities, and they knew if had not freed them they may as well have been trapped in there for enternity. So they served him during the great war until his fall, then returned to their cities to rebuild. Thats my theory 8)
Shadowlord