Things u can learn from NPCs.
I'm just writing as much as I can remember, so maybe there is some missing and wrong stuffs.
Handmaiden - Battle Precognition (You can learn it from her as soon as she joins, so it doesn't matter whether u are DS or LS.)
Hanharr - Main reason why DS is better than LS, he gives u lots of boost (1 cons, 1 wis, 2 strength, 1 char)
Mira - Nothing as far as I remember
Disciple - Learn meditation and u will get some max force point incresed.
HK47 - Learn how to kill Jedi and you will gain 1 cons, 1 wis, and a will point.
Atton - Learn how to play pazaak(after reading his mind) and you will have a will point.
T3M4 - you can fix him with some repair skill and computer skill and if you fix him fully you will have a cons point.
Mandalore - Nothing as far as I remember.
Kreia - So many things to learn from her.
GOTO - Nothing, but you can earn 3000 experience points if you find out his identity. (And some credit if you agree to deal with the planets and do it.)
Bao-Dur - Nothing for u but the droids.
Visas - Force sight. You have only one chance to ask about it, so save it before doing it.