The argument that doesn't make any sense to me is the idea that violent criminals aren't going to obey gun laws anyway, so why try to pass them? This all-or-nothing philosophy doesn't make sense to me and I don't recall hearing it proffered in opposition to any other proposed laws*. Should we legalize drunk driving since the current laws can't stop ALL drunk drivers? Should we remove the speed limit on every road because people speed anyway?
I just think that the fact that there is no magic legislation that will prevent all gun crimes should not prevent us from trying to cut down on gun crimes.
(* although I suppose I have heard a variant of it from those who would decriminalize all drugs, which I don't think is a good idea either.)