The most common reason given by those in favour of allowing people to stock up on semi-automatic weapons is the need for citizens to be prepared to fight back in case the government becomes too oppressive. It's easy to say that, I suppose it makes gun-nuts feel warm and fuzzy, thinking that someday they'll be crouching in the woods, "Red Dawn" style, taking on a rogue government.
I can't see that happening, can you? America recently went through eight years of the most fascist-like government in it's history. Americans sat by idly and watched as Bush trampled all over their civil liberties, and you all did nothing. Did any of you ever at any point give serious consideration to taking up arms? After watching Bush steal not one, but two elections, did anybody start organizing a private militia movement to take on the administration? Did we see any repeats of the Whiskey Rebellion or Harpers Ferry Rebellion?
Americans like to mouth off about "fighting back" against government when it goes too far, but that it's one of those things easier said than done. Even if the rethugs had stolen the election from Obama, the gun nuts would have complained bitterly, but done nothing.
You've got to understand that people are dying from guns EVERY SINGLE DAY, a direct result of this "gotta arm everyone" mentality. Even the most modest proposals to restrict firearms are met with howls of protest from the gun-nuts. Is this really about the need to be prepared to fight against the government, or the desire to feel manly & macho? Does anyone really need a semi-automatic weapon to protect their home and family, or to go hunting, or to do target practice?