And Bornakk basically confirms that Blizzard agrees (well, all he says is that they'll talk about it on the next Blizzcast, but he adds a smiley, so odds are that Blizzard is down with the Heroic Countenance). They've been talking for a while, too, about making the reputation grind easier for alts and guilds, so maybe this type of spell is how they'll deal with that -- get it cast on one of your characters, and it will be cast on all of them.
The only question, then, is what to do with all those Heroic keys that we've picked up. The answer is probably nothing -- Blizzard doesn't really care, it seems, about updating old content, and my guess is that the instances in Outland won't be changing much from their current form. But in Wrath, it might be fun if the keys stayed, but opened up other, less consequential content. Instead of Heroic mode, what if a Revered rep key opened up the way to an additional optional boss, or an additional loot area per run? Not enough to make it necessary, but enough to reward those who went all the way with the reputation factions in the game.