Even if you win the roll and picked a light side choice, they will still get their darkside points if they chose appropriately.
Yes, but in some cases the choices you make have actual effects. Do you destroy the kolto tanks full of insane people, or do you release them knowing it's probably going to lead to an extra battle? Sure, the other folks get their darkside points, but it might cause other problems.
There's also just a story "flow" problem. If you're in a 4-man group and three of the four are picking dark choices, then you'll go through an entire conversation where your group, as a group, is mean to the NPC and suddenly there's one nice exchange mixed in there? Even if it has no actual game effects, it's still a bit jarring.