Luke has and still does use both sides fo the force.
If you follow the EU you will find that Luke did in fact succumb to the Darkside at one point and he did so purposefully. (Dark Empire) His reasons where to better understand his enemy, and it took his Twin Sister Leia to bring him back to the light. Yet even his now wife Mara says that he has not completely turned from the Dark Path because he was using the force to impose his will on the world around him.
Individual powers (contrary to video game usage) are not inherantly Light nor Dark in and of themselves, but in how they are channeled and used. If the power comes from a negative emotional response and bends the world to your will then it falls to the darkside. If the power stems from a positive emotional response such as compassion and you allow your will to be bent by the living force, then it resides on the side of the light.
When a jedi is calm and at peace then the force flows from them.
A sith uses anger and fear to bend the force to their will.