Absent Soviet agreement to such a ban (like the one that ultimately emerged in theINF Treaty with respect to medium-range ballistic missiles), any reductions agreed to in START could be readily circumvented without detection.
Recently, the U.S. government discovered in East Germany a number of undeclared, operational launchers (reportedly 6) for theINF Treaty-limited SS-23 shorter-range missile system, together with a significant quantity of associated missiles (reportedly as many as 24).