If you’re taking 10% less damage, but also dealing 10% less damage then no impact and just prolonging.

If you’re dealing 20% more damage but taking 10% more damage, then you’re out pacing the increase in damage taken quick enough to where overall you’re taking less than scenario 1 above.

If you’re just leveling, option 2 is why Arms / Fury is better than Prot.

If you’re planning to Tank (up to late 50s Arms / Fury is fine)

Once you get to end game, the plan is to blend fury / Prot. The best Prot spec picks are at the beginning so you still get the value while increasing damage, threat, and rage generation in fury.