1) Divine Light does 14d6 per shot at maximum rank. Plus its a light-based undead-only AE and it just happens that the majority of undead are vulnerable to light-based attacks. So its more akin to rolling 28d6 for damage per turn and most of us will have 10+ turns by 14. Now if we can enhance it with the Radiance line of items... thats almost 1500 damage. Now figure using that against 4+ monsters (its an AE remember) and things really start to add up.
2) Divine Light costs 1, 2, and 3 AP for the ranks not 2, 4, 6. That makes it pretty cheap actually. Limited in targets, but hard to find 6 points that can buy around 500 to 1000 damage without spending SP or feats.
3) Get Divine Righteousness, then use that before unloading the DLs to get an undeads' attention. Good place to use it - end room in Temple of Vol if you can persuade the group to NOT drop the firewalls before the fight. Hit DR before talking to the priest, then start firing DL as soon as the vampires appear. I promise you, the main problem will be keeping alive. I've done that with my tank, and even though he only gets the second rank, he can still grab all the vampires attention even through pre-set firewalls.