So, if you buy them with any character on your account you can use them with any character on your account. So, if you had a rich Horde char, you buy them with the Horde char and use them on a level 1 Alliance. Also, if you aren't using an Heirloom and want to save bag space you can just destroy it and recreate on a specific char when you need it again.
Heirlooms are kinda over-powered, but awesome for faster leveling... I wonder if they will get nerfed with the level squish.