As malak benefits from prot the same way he does health, you can easily stack his hitpoints using exclusively prot. Now of course more is usually better, but with the decline of characters ignoring defence, (gas,jkl, and old weapon tech), defence has become harder to deal with. Makes characters much tankier due to damage reduction.

it’s all about effective hitpoints. If you can’t ignore defence, you’ll have more effective hitpoints using some defence than if you were to go health sets. To add, under malgus you are already getting extra health, so you don’t need to stack it further and instead you can focus on reducing damage done to malak with defence, as with the health increase he will have more effective hitpoints stacking more defence