One B.Net per one wow account is what I've done.

Currently you can log in with a single authenticator code.
Which is the main reason.

You lose the features of an integrated account management.
Which is new with the B.Net account thing.

So essentially your choice is:

(A)
Pro - The new features of an integrated account management.
Con - +30 second to log in, per account after the first.

(B)
Pro - Retain logging in all at once.
Con - Account management as its been for the last 4 years (ie no integrated improvements).