The libby would have to pay an extra 50 points to get 3 powers, so the Codex powers are a more economical choice all 'round. I've rarely had opportunity to cast more than 2 a turn, unless you're in a situation where Force Weapon activation is happening a lot.
No, if the Librarian chooses to exchange his powers he chooses 2 powers from the BRB, as he is Psyker Mastery level 2. He then also retains Hammerhand. This gives him a total of 3 powers available to him. You are incorrect.
I mentioned it though that it is indeed better to just keep the codex powers since they are arguably more powerful, not random and are also extremely cheap to buy more of. So unless you really wanted something from the BRB... but in such a case it's really cheap to just take a 55 point Inquisitor if that's up your alley.
Last edited by Arcane; 01-20-13 at 08:41 PM.