If you leave them all attached to the same B.Net, you get:
- easier account management, since you can switch from one account to another without logging in/out.
- a single B.Net email can be used.
- you can transfer toons between accounts on the same B.Net almost immediately.
- there are rumors of Account Bound becoming B.Net bound at some point, while this is unsubstantiated it could rock.

If you separate them into one B.Net per one WoW account:
- You have multiple usernames, which is slightly more secure vs a brute force attack should someone learn your email address used. (Not a help against being keylogged).
- You can simultaneously log in one wow account per B.Net with a single Authenticator code. This means, password + authenticator and in...... as opposed to (all on one B.Net) password + authenticator + 30 second wait, then repeat the process for account 2.. password + authenticator + another 30 second wait... (A minimum of 2.5 minutes to connect, initially and each time you disconnect).