There is also a funny interaction where leadership can become bad for you.

When you get a resistance above 100% fire starts to heal and so on.

So let's say you have 60% fire resistance and gain +50% resistance from leadership.
You then walk into fire and get burning because it will heal you and magic armor doesn't prevent good effects. However now you move away from your allie with leadership (or get teleported away) than the burning starts to dmg you.
Would you have walked through the fire with your only 60% resistance your magic armor could have blocked burning.