It misrepresents biology. It pretends to know that a particular structure is incapable of any function whatsoever if it is changed in any way. Needless to say, one would actually require complete omniscience to be able to rule out the viability of every conceivable variation. Anyone who claims that they can prove a system of even moderate complexity "Irreducible" is selling you snake oil.
That's a straw man. All irreducible complexity says is that no one can DEMONSTRATE a way the structure had any function if any of the components are removed. It doesn't generalize it to say nothing could have possibly occurred that there is no evidence for whatsover and that no one has even thought of.
Saying there could have been something for which there is no evidence and that no one has even thought of is of course non-falsifiable. It's also incredibly lame. It's also contrary to how people normally reach logical conclusions about what has happened, such as in a court of law. Consider someone on trial for murder. Let's say we can know with precision when the crime occurred, and it turns out he was being booked for anther crime 500 miles away at the exact time the crime occurred. If the prosecutor told the jury to ignore that on the grounds that there might still be a way he committed the crime even though he can't think of what it is, what would the reaction be?
Plus with respect to some things that Behe claims are irreducibly complex, like the blood clotting cascade, postulating it could have had some other function doesn't solve the problem, since lacking clotting ability is in all likelihood fatal to a species.
Science is supposed to be about evidence and probabilities. If people are going to try to refute irreducible complexity they have to demonstrate some actual scenarios and calculation of the probabilities with respect to Behe's examples. They have tried and failed.
A True Acid Test Response to Ken Miller
As Behe says in the second article:
Envisioning IC in terms of selected or unselected steps thus puts the focus on the process of trying to build the system. A big advantage, I think, is that it encourages people to pay attention to details; hopefully it would encourage really detailed scenarios by proponents of Darwinism (ones that might be checked experimentally) and discourage just-so stories that leap over many steps without comment. So with those thoughts in mind, I offer the following tentative “evolutionary” definition of irreducible complexity:An irreducibly complex evolutionary pathway is one that contains one or more unselected steps (that is, one or more necessary-but-unselected mutations). The degree of irreducible complexity is the number of unselected steps in the pathway.
That definition has the advantage of promoting research: to state clear, detailed evolutionary pathways; to measure probabilistic resources; to estimate mutation rates; to determine if a given step is selected or not. It allows for the proposal of any evolutionary scenario a Darwinist (or others) may wish to submit, asking only that it be detailed enough so that relevant parameters might be estimated. If the improbability of the pathway exceeds the available probabilistic resources (roughly the number of organisms over the relevant time in the relevant phylogenetic branch) then Darwinism is deemed an unlikely explanation and intelligent design a likely one.