Blizzard is planning to turn Emblems into a point system, similar to how Battlegrounds give Honor points. The top tier of raiding will give Valor points, while heroics and the lower tier raids will give Heroism points.
What they do with the Badges from BC depends on how far back they want to consolidate things. Wrath instances and raids will definitely be converted into the two point system, as 4.0 will change things before Cataclysm comes out, but BC is up in the air. If they do end up converting BC into points, it will likely only be the heroics and raids, and most people don't stay in Outland to run heroics at 70. Additionally, if you do try to run random heroics for the BC instances, you'll quickly outlevel them, so you can't stockpile very much.
As for higher level players going back and farming easy heroics for points, there's the fact that they're also missing out on the gear and useful items that would be dropping from appropriate level dungeons. Plus the fact that a good deal of classes could probably solo Wrath heroics at 85 anyway.
All in all, I'd say it's likely that Badges will be turned into points, giving you some small head start towards a piece of gear at 85. On the off chance they decide not to convert them, there's always uncut epic gems. They'll sell on the auction house to people leveling jewelcrafting if nothing else.