I've use this spell in combination w/Grip of Winter while inside a castle. My banefire archers & mages (protected from cold) mowed down the opposing army from C'tis while a coupld of thugs & assorted chaff held the gate.
Yeah my first thought is a storm-defense as well. Slow down the enemy and your own men, which works to your advantage here since they wont run out. Gives you more time to pepper the enemy and the possibility of getting multiple short-range spells attacks (basically the only time they can ever be used reliably).
Quagmire is useful for battlefield destroying spells or bunches of mages. Slowing down the enemy while they take hits from spells just means that your troops take less friendly fire.