RDM: Does it get fun at 70?

[Tzain]

Um...its actually very useful.

You keep impact in reserve for when Verfire or Verstone doesn't proc to keep your potency up. It has the same potency as those spells, which is 30 more then Jolt 2. Your main goal as RDM is to try and use the stock Jolt as little as possible, and Impact adds a very nice tool that helps keep your damage up.

You seem to be confusing the 'design' of impact with the 'implementation' of impact.
Impact works exactly as designed, exactly as you said. So you are speaking of the 'implementation'.
Zfz is speaking on the design itself. The 'Why' of the skill, if you will.
And as a design, Impact is a head scratcher.