The reason you were assigned the wrong role is because you play a class that can spec to more than one role, e.g. Sage/Sorcerer, Powertech/Vanguard, Scoundrel/Operative, etc. You are asking Bioware to code group finder so that it can somehow "read" your skill tree and determine your role. What would be easier is to simply check your desired role(s) and uncheck the undesired ones on the group finder window.
Sorry, but not true. Each time I start the group finder I deselect DPS. But twice I have been placed in a group as DPS. What you describe is how it should work. But in reality it makes mistakes. I'm not asking it to read my skill tree. I am asking it to honor my choices.