Converting them into honor to complete my pvp set :P
Pretty much this.
When I hit ~4k JP, I dump them all into honor. On my main that has better gear, I'm buying the PvP mounts (Just have the black mechastrider left to get), and on alts I buy PvP gear.
JP is basically worthless as actual JP in Mists, which is pretty sad. And heirloom upgrades to get from 80 to 85 for about the same cost as it originally took to buy the 1-80 piece? And they couldn't even make it go to 90? Well, considering I have 7 classes at 90, 1 88, and 3 85's, if they can't even bother to make them go to 90, then I guess I won't be wasting JP on that.