Attaching a Blizzard Authenticator to your Battle.NET account gave you some stupid Core Hound pet in World of Warcraft. Other than the greatly enhanced security, are there any similar (trite, cosmetic) benefits in Diablo III?
Not at this time.
At least I have not received any for my previously purchased authenticator (that I bought for WoW).
I should also note that the core hound I got for my authenticator in WoW was bind on account, so it was available across all of my WoW characters and their servers. If something were done in Diablo III, like the angel wings and dyes for purchasing the collector's edition, it would probably be the same way.
How about the benefit of not getting hacked? I tried to go without it and I swear my account got hacked at least once a month. I am kind of adverse to the idea (as I use the android app) because I re-flash my phone a lot and it is easy to forget to backup the app data. Now that they have implemented a restore key, I feel much better about it and I am using it. Seriously.. Since D3 came out, my account has ALREADY been hacked. That was the last straw, seeing 2 characters on my account that I had not created and no one else has access to my account.