A conventional ICBM or SLBM muddies the waters and makes things even more dangerous. Just because a country says that they don't have a nuke doesn't mean that they're telling the truth....even if they sign a treaty. It also doesn't mean that they don't have alliances with nuclear capable states. Let's say, hypothetically, that we launch one of these things at North Korea who has sworn that they have given up all of their nukes and they retaliate or China retaliates for them? What have we accomplished? That's problem #1. Problem #2 is that some moron will be less than contemplative about launching one of these things because they are "less lethal" and start off a firestorm first from the confusion created by problem #1 and then by the rightful anger of the target, their allies, and anyone else that is pissed off by such a stupid stunt.