OK. Regarding merging Blizzard/BNet accounts or moving WoW licenses from one Bnet to another... it just cannot be done in a way that makes sense for the player, for seasoned accounts.. The pool of Mounts/Pets/Toys doesn't get merged, but would merely inherit the Blizzard account items that licenses would move to.

So.... if you are starting out... keep everything on as few Blizzard accounts as you can (8 WoW license limit per Blizzard account) as MiRai says.
If you are adding new accounts... yeah, add them to you existing account(s).

For me? I'm pretty much out of luck. My "main" account is missing some key mounts so moving WoWs over would be a loss for me.

Also, they are trying to called main account/launcher "Blizzard" now instead of "Battlenet"... which make things harder to talk about, IMO.