Jeff Cooper used to say, "It is long been a principle of ours that one is no more armed because he has possession of a firearm than he is a musician because he owns a piano. There is no point in having a gun if you are not capable of using it skillfully."
And I've seen plenty of atrocious gun handling and abysmal marksmanship at the various ranges I frequent. I've seen a lot of folks who apparently have had their guns for a while and who couldn't put two successive shots within 15 inches of each other at 7 yards. Aside from the hazard such people might pose to others, if they needed to use their guns in self defense, they might not be able to do themselves much good.
There probably isn't such a thing as too much training or too much practice -- as long as it's good training and good practice. And I think a good foundation would be the three NRA courses -- Basic Handgun, Personal Protection Outside the Home and Personal Protection Inside the Home. If properly taught, the three together will cover: basic safety and shooting skills; basic self defense shooting skills, including combat accuracy, flash sigh picture, basic point shooting, presenting a gun from a holster, speed reloads, basic movement and use of cover; equipment selection and use; legal issues; mind set and attitude; situational awareness; and dealing with the aftermath of a self defense shooting.
I'm not saying that sort of thing should be a requirement. But if I'm going to be carrying a loaded gun around and think that I may possibly need to use it to defend myself or a member of my family, that's the sort of stuff I'd want to know about.