The bottom line, IMO, is that a distinction between assault rifles and sporting or hunting rifles is irrelevant, because it's a constitutionally protected right for a citizen to own and bear either.
To say a citizen "shouldn't need" one because of it's intended use, which is stated to be killing other human beings, is only applicable to military personnel is incorrect on several fronts. First of all, citizen participation in national defense is legitimate, probably more so than standing military. Second, I'd say killing other human beings is not the intended use. I'd say stopping human aggressors is the intended use, as that's generally the justification for warfare, right? In that case, the same applies to self defense as it does to warfare (in my opinion, with principles of liberty in mind), and in a case of self defense, it's legitimate to use a tool designed for the job. Why would you try to use a tool to stop a human aggressor that was designed for harvesting deer? That makes no sense. To stop a human aggressor, use a tool that was designed to stop a human aggressor.
I too have heard of at least one instance of home defense using a semi-auto rifle. It was actually a YouTube video of the home owner's security cameras. Never saw an official report of any kind, so I just assume it was legit self/home defense.