I had five accounts, spread across five B.Net accounts.

Then consolidated down to a single B.Net.

And later decided to 10-box, which obviously meant more than the 8 account cap per B.Net.
Using the web chat based support, they had no issues moving two of my original accounts off of the one B.Net and onto one of the previous B.Net emails.

So, it is likely that you can both consolidate and split your accounts at will.
But that is not a guarantee.



The biggest benefits are:
a) One authenticator, for your entire B.Net... which could be eight accounts.
b) One www.worldofwarcraft.com log-in, for account management of every account.
c) Shared achievements/mounts, across the entire B.Net for many things.
d) Instant mail between your own accounts, even if they're not in the same guild.

And the biggest detractors:
a) You're limited to one beta account, per B.Net email.
b) Cross faction arbitrage, via the Neutral Auction House, isn't nearly as easy.
c) Pet battle issues.