I live is Canada, so I'm certainly not an expert on less restrictive gun laws. I wonder about this however. I would think that even with laws that permit citizens to own and carry a weapon (concealed or otherwise), in a situation like in Denver, the ability to carry and shoot (say the type of practice a gun owner would get at a shooting range) would be useless as this is a completely different ballgame from controlled target practice.
My major concern any argument for carrying a weapon is that I do not think that the average gun owner (i.e. one that practices in controlled shooting situations and does not have any situational or combat training) would have been effective in stopping a shooter like this. One can extend this further and ask if even in a one-on-one firefight (which again, is not a scenario that I would imagine a typical gun owner trains for), an average gun owner could not reliably be expected to assess the situation and determine if shooting back would be a good or bad idea.
All in all, I think that while denying the ability for a citizen to arm themselves may be putting the weapons in the wrong hands, putting weapons in the hands of people that are not trained to use them effectively is not the full solution either. Perhaps having different types of weapon permits, one for hunting, another for shooting practice at ranges only, and carry permits that require mandatory combat and tactical training along with re-certification (like in the armed forces or law enforcement) would be a better idea.