In short, the developers agree that the process of mailing heirlooms around constantly is a little annoying. We'd prefer to try and work ourselves away from the method of mailing out BoA items in the future, like pets, mounts, etc. To that end, we have some design goals in mind that would improve this sort of item distribution system. And with those design goals have come some ideas for changing heirloom mechanics. This is sort of contrary to the idea that was shared in the past about just designing the system to allow players to mail their heirlooms all around their accounts without realm restrictions.
That's all of the information we can share at the moment, but we feel like it's worth pointing out that the design philosophy we shared earlier isn't so set in stone -- we're reevaluating these types of systems as we look toward the future. World of Warcraft is a living evolving world, and so the systems and mechanics that make it possible.