As you might expect, monsters of the Spellcaster family tend to cast spells. The Rune Blade is effective against them, and unlike most family-killing weapons, this one has a few very handy strengths. In particular, it's an excellent weapon to use against Ogre Mages, as the accuracy bonus helps break through their evasion when they cast Blinkspells. It can also be a useful weapon against Dark Wizards and the Evil Eye in the Ice Cavern, but beyond that point in the game, it loses a lot of its appeal.