It depends, the Banshee can be used for either situation.
Against masses of weaker enemy units that aren't worth stealing, Curse can do wonders. You're basically giving your units greater evasion than even a level 3 Evasion can offer. Banshees make excellent support units here, and are effective at taking the edge off of enemy attacks. Great stuff in mirror matches, as Ghouls/Banshees can mop the floor with just plain Ghouls.
Against heavier enemies, Possession. 'Nuff said.
Against casters, or hard hitting AoE spells, AMS.
The Banshee really makes for an excellent support unit, as it is versatile and powerful. Banshees make for great stuff to add into armies and I do my best to add them whenever possible. Even Curse is just too good to pass up.