I wouldn't be surprised if they do this eventually. We now know that Blizzard has had their hands
very full with the Patch To End All Patches, so I can see why it hasn't come yet. Hey, maybe it'll get tacked onto 2.1 though, right after they make Gruul drop "The Kitchen Sink"?
As someone said in another thread, though, I wonder how they'd balance it within the framework of existing heroics. Many of the old-world 5 mans have no associated rep, or are impractical to gain (like Shen'Dralar in Dire Maul). Plus, some of the obvious reps are Alliance or Horde specific, like Gnomregan. Would they just eliminate keys for the old instances? In that case, some people would just farm them instead of bothering to rep up for the current heroics. Maybe make it something arbitrary, like revered or exalted with all 5 of the current heroic factions? Would the loot be better, sidegrade, or worse compared to the current heroics? You wouldn't get me within 100 miles of Heroic Maraudon if the loot wasn't up to par.
It's a fun concept, but there are some implementation details to work through. I'd run a few just to see the places again (Deadmines, Shadowfang) but most of the old instances would need amazing loot to lure me in.