Well, you can influence and change your NPCs.
Yes, you can say the wrong things and get them to shut themselves off to you.
Just like real life.
If you bully them, you're probably not going to get too far with many of the party members.
Just like real life.
If you tell party members what they want to hear, and twist upon their existing feelings, you can change them. I've personally watched light side characters become dark side. I've watched NPCs that used to bitch about evil deeds gladly take part in them, and comment accordingly.
What more did you want?
Personally, I think the influence system went over a few people's heads on the first play through.