Its defineately worth alloting "attacking" skill points to defenders / goalkeepers, and likewise adding "defence" points to attackers.
What good is having a defender or 'keeper, who dosen't know how to distribute the ball, because he hasn't been trained in the attacking art of passing or dribbling.
Likewise, an attacker is less efficient if he hasn't learnt the art of heading, positioning or marking, that can only be gained through the defensive trait.
Every player needs to learn at least some of the basics of the other positions to which he is not accustomed.