I think that it's very important to practice full power strikes on a heavy bag or at least on a kicking shield. My sensei was going over this the other day with us in class. The point being that if you don't work your technique at full power at some point it won't be there if you need to defend your self. I can imagine how bad it would be to have to strike someone in self-defense and your wrists weren't strengthened through full power striking at some point in your training.
If we practice punching air and don't really hit anything, how are we ever going to develop the power needed. I have seen people perform forms who looked great. Their punches and kicks were crisp and appeared to have alot behind them. Those same people when striking a heavy bag or kicking shield are a mere shell of their self as far as striking power.
I guess it all comes down to this, If you are taking Karate you got to hit things. There really isn't any way to get around it. we can't punch air and say, "that would stop an attacker." We have to build our power through repitition.....we got to hit stuff.

keep training, Mark