There's actually quite a bit of trouble that could come up, but we know they have the ability to do it just fine considering server transfers don't cause you to lose all your items. Question is, how would this "kill off server transfers?" Do that many people pay to send over a toon holding a couple sets of heirlooms?
That aside, I still think they should just make it so when you buy an heirloom you're buying a "license" account wide, so any toon of yours that goes to a vendor can receive that specific heirloom anytime they want. (Possibly with a 24 hour cooldown.)