Nice strawman, but that isn't the same thing. HP recovered and damage prevented occupy two different spaces in the absolute economy of the party, though I can understand how some might think otherwise.
In the previous example, saving the fighter via damage prevention would have been better earlier, since they'd get more hp benefit. Saving, for example, 10hp, the fighter would be at 15 and go down to 9 when hit, instead of being at 6hp and staying there.
However, if the power was used to protect the rogue, it wouldn't have been used on the fighter, who ended up taking enough damage to drop him.
That's the basic argument ... when used early, it's harder to know if it will matter in the long run. You may have prevented someone needing to spend a surge after the encounter is over, but preventing someone from dropping (or becoming bloodied) can have a more substantial "in" encounter effect.
Then again, if the attack does something else on a hit (immobilize, stun, daze, etc) it is probably worth making it hit regardless of how much damage it is going to do (unless it's say, a warden, who will easily shake off the condition compared to someone else that may also be hit with the same attack).
