Kings Defender is not a paladin tanking weapon (well, in the current game anyway). You want a spell damage weapon.

There are some limited occasions when you're off-tanking a mob and it's not being DPS'ed and you don't have to worry about threat as much where a little extra mitigation/avoidance would be great for your healer. Suneater is a good example and what I use. But, there weren't many times I used that while raiding (Phase 1 A'lar and FLK add are the only two I can recall atm).

Of course, come expansion, things are going to change a great deal so who knows what you'll need.