Barthundra is essentially "Barstunra" and it works against physical types of stun too. For example, if you cast Barthundra/Barthunder and are hit by the Yagudo TP move Sweep (AoE dmg and Stun effect), you have a greater chance of resisting the effect or the stun effect not lasting as long. I can even see it working as DRG/WHM in Campaign.
All physical moves with added effects (Delta Thrust, Violent Flourish, Sweep, Frypan etc.) work with and factor in magical resistance to decide whether or not the effect will process.
SE really don't have a reason for not adding dark and light resistance spells to WHM and RDM, same reason they never had a reason to hold back Enlight and Endark until recently. I still don't agree with those two spells being given to PLD and DRK respectively, but we really do need light and dark resistance spells as WHM and RDM.