A lack of proficiency, IMO is only a danger to themselves
The canary in the coalmine so to speak - that brings to light the existance of a situation of an idiot with guns - are kids.
That's why we have kids going trick or treating with a real 9mm.
My kids are training in firearms safety. I think one of the most important things for a child to see is what bullets do to objects... then it's no longer a video game or a movie but they have a feeling (hopefully) for the destructive capability of a firearm.
But anyway, most of the time when there is a child involved in a shooting there is an ignorant and untrained gun owner who owns the weapon.
I guess you're right about mostly being a danger to themselves otherwise - I sort of think of Plaxico Buress type incidents - or like the guy who just shot himself at Walmart last week. I guess history shows they are more likely to hurt themselves but logically I don't see why they're not a danger to the general public.