According to Behe there is no imaginable way that the necessary combination of molecules could be built up piecemeal, as the theory of evolution requires; either they are all present, or the process does not work.
Most biochemists believe do not believe that the concept is useful, because Behe ignores mechanisms by which complexity comes into being. One such mechanism is "scaffolding" as a set of biochemical reactions are used to build up a pathway and then are discarded, in much the same way that a building is built from the botton up even though removing any of the columns would cause the building to collapse. Another mechanism is that a set of biochemical reactions start being used with something completely unrelated. For example, the fact that any step of light detection would render the pathway useless for light detection would not be important if the original adaptive benefit of the pathway was something unrelated.
See: intelligent design