This is actually an effective argument for
comprehensive gun control. An assault weapons ban would not prevent those two mass killings you cite. But, however, having a comprehensive system where, for example, mental health records are tied to gun purchase background checks would have prevented Seung-Hui Cho from purchasing handguns.
Gun control is not just one policy. It's a broad policy with many possible facets: an assault weapons ban, federal standards for background checks, cracking down on sales at gun shows, eliminating loopholes, tying mental health records to background checks, the list goes on.
EDITS (reply to below, additional things constantly added):
I'm not naive enough to say that comprehensive gun control is going to eliminate all gun violence. I'm saying that it's a step forwards towards reducing gun violence.
>We arguably have comprehensive drug control. How well does that work?
This is a non sequitur. Just because the drug war has failed and the phrase "comprehensive drug control" shares two words with the phrase "comprehensive gun control" is not enough to logically lead to the conclusion that gun control is/will be a failure. The comparison needs to be elaborated. Let me elaborate on a few differences: people don't get addicted to guns like they do with drugs, someone using a drug can not directly physically injure someone else, drugs are much smaller and easier to hide (EDIT: for smuggling on an individual basis, not for a cartel), guns are hard to mass produce without people noticing. Drunk driving is not really relevant unless there is a large trend of accidents caused by cocaine/heroin/marijuana (rather than just alcohol). Guns are much less profitable for cartels than drugs are (for reasons of addiction).
I think nitpicking over the specifics of an assault weapons ban does not really counter my point. I meant semi-automatic weapons with magazines/cartridges (and automatic weapons). Even if my definition misses something, I'd be willing to just concede that and expand my definition. Yes, it's obvious that a ban implemented today would not stop Adam Lanza, because the guns are already bought. It might reduce the number of deaths of an Adam Lanza 5 years from now (I'm acknowledging of course that he also had two handguns with him).
To be clear, I don't propose that we should have a blanket handgun ban. It's simply not a realistic goal (politically and in terms of all the guns already out there). There are ways to improve gun control without banning all handguns.
I also think that the mental health system in America is broken and needs fixing. The two things are not mutually exclusive.