It does provide a check -- not a definitive one, but one nonetheless -- against government encroachment on peoples' rights, [...]
So, would you say, the US government hasn't encroached on peoples' rights since 9/11?
This isn't aimed at you specifically but I have to admit I always have to chuckle whenever this argument comes up because I think it's amusing how people imagine totalitarian regimes to come about.
Usually, governments don't decide to become evil overnight. It's not like it's a liberal democracy one day, the Fourth Reich the next. It's a process, and usually, a large number of people go along with or even support it for various reasons.
So even in an armed society, only few people would fight the regime, they'd be terrorists, and terrorists have a way of getting weapons, even in countries that don't grant the right to bear arms like the US does. So, it wouldn't make much difference, anyway.
In the end, for most people, living in a totalitarian regime isn't that horrible, so long as their material needs are satisfied. The vast majority of my fellow countrymen continued to live their ordinary lives in the Third Reich, at least until the war. They didn't particularly miss the freedoms of the Weimar Republic. In fact, many of them later missed the Third Reich.
So, it's a nice illusion to believe there would be an armed mass uprising, but it's an illusion nonetheless. The only way to fight encroachment on our rights is to fight every little one by legal means, by protesting, by raising awareness in the media. Thankfully, for all of us living in liberal democracies, these things are possible.