It is absolutely correct that this bill would not likely have prevented any mass shooting tragedies. In fact, if you go back to my very first post in this long long thread, I wrote that none of the gun control measures being considered would have prevented Newtown. Newtown served to focus attention on an issue that was well worth discussing, but nothing being proposed would have stopped it.
That being said, I strongly believe that universal background checks would decrease gun violence in general in this country, and perhaps significantly. I have made this argument several times in this thread, but for those who have missed it, here it is again:
First off, we have no idea how many private purchases and transfers of guns, which currently do not require a background check, are illegal sales (meaning the purchaser is either a convicted felon or mentally ill). Law enforcement suspects that a large percentage of bad guys acquire guns in this fashion, but we don't know (and never will) what that percentage is. Right now, for an illegal sale to happen, only one party needs to break the law: the buyer. The buyer simply does not inform the seller that he, the buyer, is a convicted felon. The seller, under no obligation to make a background check, doesn't know any better. He simply sells his gun for cash and that is the end of the transaction. And this happens all the time at gun shows.
But if we require universal background checks, in order for an illegal sale to happen, TWO parties will need to break the law: both the buyer AND the seller. Even if the buyer lies about who he is; the seller will have to either neglect to run the background check, or be willing to sell his weapon to a felon anyhow. I believe that most gun sellers are honest and do not deliberately want to sell their weapons to felons or crazy people. So they won't. Which means it will be that much more difficult for the bad guys to acquire guns. Impossible? No. But more difficult, yes. And that is the key to effective laws.
So if those opposed to this bill tell you that it failed because it would have been ineffective, they are wrong. It would have been quite effective. This bill failed because the NRA successfully convinced it's supporters, who successfully convinced their Senators, that this bill was the first step in a concerted government effort to seize all private firearms.