Actually, a complex molecular system can be built gradually if the pieces
are available. For example, a complex system that makes compound A into
compound E could be built up starting from something that makes D into E in
one step, adding something that makes C into D... Also, the assembled
pieces need not be single-step. The citric acid cycle can be split into
two complex parts that function independently in some bacteria, for
example.
It should also be noted that Behe advocates intelligent design of the first
cell with ordinary evolution thereafter (although he will not personally
rule out the possibility that the design was built into the laws and
structure of the universe and "naturally" developed into the cell). This
is a much larger role for evolution than is allowed by many of those
invoking Behe as proof of their views. However, as Dennis Lamoureau points
out, it is exactly the same role for evolution as that advocated in The
Origin of Species by Darwin.
David C.