It should be as follows: there are three trees. healing, damaging, and tanking. from any tree, you can pick any skill currently unlocked to you. all the skills are unlocked as you level, three opening up every time you hit a certain level. Lets say your an operative, and you follow the healing tree a little bit. you learn the skill empathy (same as engineers) and you can be a dmage dealer, and also a healer. and so on and so on. this would allow many more possiblities to the aspects of combat.