Departure angle affects what kind of angles you can drive off of (or drive on to in reverse). ANY extended shackle hangs down lower than a stock shackle in the rear, which will cause you to get caught up on more objects. Just look at any side-angle picture of an X with Warrior 153 shackles on it, you will notice the shackles hanging real low right behind the rear tire.
About effecting the springs. Shackles should sit at a 45* angle. A longer shackle causes the spring to sit higher (creating lift), but it also causes the leafs to sit longer, which isn't what the leafs were meant to do. If your leafs are a little on the weak side, longer shackles will just encourage sagging. If the leafs are strong, the shackles will sit at an angle less than 45* which lessens the usefulness of the shackle.
I, personally, am not a fan of any kind of extended shackle when running stock leafs, but something about 3" longer (like a Warrior 153) won't hurt but so much. I would stay away from anything longer though!