You'll want to keep two weapons, one for most cases with lots of spell damage and another with as much stamina and avoidance as possible for tough boss fights where you don't have to worry about threat (Such as Prince Phase 2, Netherspite). The Kings Defender is an excellent choice for when you don't need to worry about threat.

I'll switch my weapon mid fight if my health is fluctuating alot but my threat is blowing away the DPS. Otherwise, keep your spell damage weapon equiped..