I wil just say, adv job=another job.

WHM is good with /BLM or /SMN

/BLM offers:

Escape
More enfeeble
Warp
Drain/Aspir
more MP
Elemental Seal
Conserve MP

/SMN offers:

More MP
Auto Refresh.

Mainly the /SMN here is fine because /BLM is half its power(healing power is differnet, hence /WHM is ALWAYS viable for mages). Elemental and Dark is half, so ulitility spells are useful here, and why /SMN is ok too.