That doesn't mean that we shouldn't be willing to fight if our rights are threatened. The Nazis wiped out dissent rather well, but it didn't stop some brave people.
Honestly, it should never come to that, but the simple inconvenience of an armed populace has been helpful in the past(recall the Imperial Japanese unwillingness to commit to trying to invade the continental U.S.). I'm not saying that gun rights completely prevented Japan from attacking in such a way(there were other considerations, like the actual military response), but just having it factor into things creates some level of deterrence.
Besides, look at Iraq. As recent history has demonstrated, low-tech insurrections can still be a major pain to deal with. Unlike Iraq, where most of the weapons were old Soviet gear and ducktape, an American rebellion would benefit from things like air-conditioning, the internet, and Home Depot, along with the ability to maybe strike from along the notoriously insecure Mexican border. Such a revolution would not have to actually win and take D.C., but just make themselves a horrendous nuisance to the powers that be. Being such a potential pain in the ass is deterrence to tyrants, at least somewhat. There will certainly be no lazy dictators arising in the States.
Besides9again), you know that some rednecks up in the Tennessee hills are just waiting to take their revenge on the Union and get their slaves back.
They wouldn't even care about the politics of rebellion, they'd just want to wave rebel flags and shout angrily at Billy Yank.