Well, there is the approach of swapping over to a tanking sword once you have a sizable threat lead. This will lower overall damage done to you, making it easier for the healers.
In terms of using a non-spell damage sword for the entire fight, honestly, if you're going up against anything that's truly spell immune, you have far greater issues than what sword to use, because as a paladin our entire ability to generate threat is based upon magic.
Personally I can't think of anything that would call for just using a tanking sword if your an Alliance player. But mixing it up is a good idea.
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