During the recent gun control debates I've seen a number of versions of this claim and response:
Gun rights advocate: "It's senseless to put gun laws/universal background checks in place because criminals won't obey them anyway. They'll evade the laws/checks and get guns anyway."
Gun control advocate: "That's a stupid argument. We know that criminals will ignore laws about murder, bank robbery, burglary, etc., but that doesn't stop us from passing legislation on those matters."
This is shoddy reasoning on the part of the gun control advocate. There's an important disanalogy between gun laws and laws against theft. The latter laws address things that are inherently immoral, i.e., it is clearly morally wrong to rob a bank and such robberies have a deleterious effect on society. The act itself is worthy of prevention. There's nothing wrong, inherently, with owning a gun. We regulate guns because we're concerned about their being used to commit crimes, not because any and all gun ownership is immoral and has societal harmful effects. That guns have a potential to increase violent crime is a legitimate reason for regulation but it's also an important difference when assessing the legitimacy of legislation. If our ultimate goal is to prevent the implementation of guns for bad ends rather than simply preventing gun ownership itself, it's reasonable to consider whether the legislation will work to prevent the instances of gun ownership most likely to have the effects about which we're most concerned. That's why it's reasonable to assess gun control measure on the basis of whether or not it affects those who are most likely to misuse guns.
None of this is to argue against gun control. I think that as a matter of fact controlling guns is entirely legitimate and prudent, mostly because I think gun control does in fact help decrease gun ownership amongst criminals as well. My point here is to note that gun control advocate are performing flawed reasoning when they assess gun control legislation in the same way that they assess legislation against things more directly immoral and/or having a direct negative consequence on society. The right response to the argument "If we outlaw guns, only outlaws will have guns" should be to ensure that that's not the case, not deem such considerations irrelevant to the question of whether or not we should try to outlaw guns as the argument above seems to do.