With Patch 15 Blizzard has now enabled The Authenticator when you login. This is to test the system but it will stop people that rent out their Beta Accounts or share accounts if an authenticator has been activated. You don’t have to rush out and buy one now or use the free mobile authenticator, it only applies if you have activated one.
When the game is out on May 15 I suggest you do use one, either the dongle or the mobile authenticator to protect your account.
Please note that, after your first successful login with an Authenticator, you may not always receive an authenticator prompt when logging into the game. Not to worry! This is an intended convenience feature and doesn’t represent a risk to your security. If you’re logging in consistently from the same location, you may not be prompted for an authenticator code.