There appear to be two arguments being made against KJVOnlyism.
1. It is better to presuppose that only the original autographs are inerrant, and try to reconstruct them by scientific investigation, than to presuppose that the King James Bible only is inerrant.
2. It is not necessary to have an inerrant Bible, since people can be saved and grow in the faith by reading ones that only have the doctrines that I consider important intact.
These arguments, though contradictory, are often both asserted by the same person.