1) I care because a lot of guns are acquired by criminals from honest people, either knowingly/negligently (gun shows without background checks being the obvious example) or inadvertently (lost/stolen). Reducing the legal boundaries of plastic/undetectable guns means that there are going to be fewer of these and a correspondingly lower probability these guns will be widely distributed or used.
2) The "if a criminal really wants to" argument assumes that all crimes are carefully planned and executed, when facts show otherwise. Statistically, most crimes are unplanned, opportunistic, and random. The chances of all kinds guns (plastic/undetectable being one subset) being used is lowered when access is reduced. Again, facts and statistics bear this out in places where gun control limits access and there is correspondingly lower rates of run related crimes.
Overall, bans like this do have a simple, logical effect of reducing crime. In concert with other measures, laws in general reduce the probability of potential actors from simultaneously having the means (anti-organized crime, gun bans), motive (anti-poverty measures, education, penal), and opportunity (police patrols, security systems) to commit crime. No single thing with STOP all crime for all time.