ID and NS: tit for tat

Proponents of ID argue by trying to establish that NS is not sufficient to explain how the many species now extent on the earth have arisen from simpler life forms. Some proponents of NS claim that any such argument is illegitimate unless its proponent also provides a plausible alternative explanation of how evolution has taken place. But if the same proponent of NS should also argue that abiogenesis is the result of chance and necessity (rather than design), then it wouldn't this claim likewise be illegitimate unless its proponent provide a plausible explanation of how abiogenesis has taken place?