This argument usually goes with passionate invocations of Occam’s Razor. Well, echoing Einstein, the answer is very easy: nothing is really simple, if it does not work.
Occam’s Razor is certainly not intended to promote false – thus, simplistic — theories in the name of their supposed “simplicity.” If Darwinian Evolution and ID both explained well what we know about complex biological information, then we could argue about which is the simplest theory. But that’s not the case. One of the most important results of ID theory is that it effectively falsifies Darwinian theory.
We should prefer a working theory to a falsified one, without arguing about “simplicity”.
Moreover, ID and Darwinian evolution are so different that it is really meaningless to compare their “simplicity.” According to Darwinists, ID is “not simple” because it postulates a designer. According to IDists, Darwinian Evolution is “not simple” because it tries and fails to explain complex biological information — which has all the key properties of known designed things — through complicated, ad hoc and artificial assumptions and question-begging rules, just to avoid the “simple” (and even, “natural’) explanation of a designer.
Such discussions are really pointless, more philosophy than science. The only important scientific point is: which theory gives the empirically well-supported, “best explanation”?