Prot Warrior - Why Heroic Strike at all?

I'm recently playing my alt warrior as a tank in leisure time. Every guide I read suggests that once the buff "Ultimatum" is triggered, we can make use of heroic strike for more dps. However, in my current gear level (453), heroic strike hits less than devastate, not to mention that devastate also has a 30% chance to reset shield slam. I haven't tanked raid so no idea about what happens in high vengence level, but I doubt heroic strike is worth using even then. Am I missing something here? Because if I'm right, then what's the meaning of this skill "Ultimatum"?

Heroic strike/cleave are off the GCD, so using them has no impact on your ability to devastate.

I see. Many thanks!