Disclaimer. The thoughts expressed in this page have not been explored very deeply by yours truly so they should be considered with a grain of salt. However I believe that the general idea is, if nothing else, interesting enough to be out there. It is possible (but not certain) that I'll research more on the subject in the future; if that happens it is possible (but not certain) that I'll update this page to contain more refined opinions.
Scenario: the government does something that makes a considerable amount of citizens very mad. It is natural for such citizens then to organize a protest of some kind. However such protests are rarely succesful and, if they get too problematic, then the government can simply ask the police to go and dismiss it; by the use of force if needed. In states where citizens do not have the right to keep and bear arms (or in the states where this right is granted, but the process of aquiring a weapon is too much of a pain for the common folk) the amount of force required to quiet a problematic insurrection is clearly less than what is required in states that grant such a right. For example it might be enough for the police to dissipate the crowd with jets of water or tear gas and, if worse comes to worse, there's always the nightstick. My claim is that this is not a plus.
Imagine now that the threat of a considerable amount of people at the protest bearing some sort of fireweapon was real. Then the police force (or whoever does the government's biddings) will have to bear similar weapons as well: jets of water and gas aren't as effective if there is a real possibility for shots to be fired as soon as vans and weird tubes are spotted. Of course the bugman will say "but how can this be a good thing? The policemen are trained to use firearms. There is no way that the common people can put up a fight", but this is both precisely what I want and what the people in power don't. Let me explain myself. Water jets and the such are a great way of dismissing a manifestation because not only they physically stop people but also because the image of such acts does not bother public opinion too much. Nobody died so the ones that were trusting the government before will most likely keep trusting it afterwards. However once there is blood on the line, once someone gets killed by the police (because of the training on the use of firearms and because firearms must be carried as a considerable number of people might have them too) then the government will immediately lose the trust of a much much bigger portion of population.
Without the right to keep and bear arms the government has much more freedom of action. It can safely screw over even big numbers of people because as long as these numbers aren't too big protesting will not decrease trust (and thus power). On the other hand the right to keep and bear arms keeps the goverment in check: motions that will screw over less people now come with the possibility of causing riots that cannot be dismissed without spilling blood i.e. without necessarily losing power.