I think WHM is the natural choice to add damaging light-based spells to. They should never be up to BLM nuking strength on normal enemies, but on undeads I think they should be pretty close or equal to them.

Of course, WHMs wanting to compete with BLMs for damage on undeads should also be required to stack up on about as much nuking gear as a BLM normally does. Nuking in Curing Potency gear clearly shouldn't let them play at BLMs level.