My experience, it’s not a grain of salt nor a bucket of sand–that’s an entire beach.
The btag system was integrated after WoW account and subscription systems. For example, you may have one active subscription on WoW Account 1 but not on WoW Account 2 all under the same btag.
You’re right that it should be simple to maintain functionality in the general case–one btag is one account. I doubt there would be a lot of exceptional cases.
But it’s Blizzard we’re talking about and from what I understand these forums aren’t directly “owned” by a dedicated Blizzard-specific group but a series of contracts held by Blizzard. Add in a couple layers of obfuscation and “discovery costs” in a formal request for proposal, and you have a lot of people suddenly saying “Let’s just leave that sealed because it works well enough and continues to deliver some marginal value.”