If you had say, a Rumble 200 Combo with a Rumble 15" extension cab. I would think a HPF still be useful for taming booming when necessary but wonder if you would still have to worry about cranking it? Unless, the HPF causes the amp to exceed the ratings for the speakers?
I deleted my reply to think about this, but the hpf helps your speakers becomes 'more efficient' and not increase amp or speaker ratings...I think.
Using the HPF *could* allow you to drive your speakers harder than they were designed for, since you might not hear them farting out, nor have the visual cues of the speaker moving more than is healthy for it.
.