Looking for the Value in Value-Added.

In all of the argument and debate in the merit-pay/ teachers suck arena, the topic of value-added assessment comes up often. I wonder if anyone can actually delineate in which fashion or way that a multiple-choice, standardized test that is taken by students who have no pressure on them at all and who have no immediate personal need to pass the thing can possibly measure the educational state of said students, let alone any "added value" imparted by me.

Standard-based, constructivist, traditional ... none of that seems to matter.