Regardless of what you all have said... I'm an engineer at Rensselaer Polytechnic Institute, and the answer is that the decrease in for has to do with impulse, or forceXtime. When climbing companies rate equipment in terms of kN, this is a force. This force is the maximum allowable force on the piece. If, for example the rating is for 6kn, and the piece holds for 1 second, the resulting change in momentum is 6kN sec, which can be translated to a change in momentum. To summarize, no matter what the piece is rated for, it matters how long it holds. A 12kN piece that holds for 50% the time of a 6kN piece changes the force on the other pieces exactly the same amount. More pro is better in general, and everything takes energy out of the system, even the friction of a cordelette over a biner attached to a weak piece.
Also, for the mathematicians, the change in momentum is equal to the integral of the force in terms of time, and since momentum is mass times velocity, the change in velocity is the integral of Fdv/m.