If one truly wanted to get past the checkpoints and not fly, they could always buy a fully-refundable Y fare ticket, if they're not concerned about a large charge on their credit card appearing before it's reversed. So this capability has already existed.
It has never bothered me in the past when non-ticketed customers could pass through security. I used to do that plenty of times, when picking someone up. That sure beats hanging around LAX's dreadful baggage claim areas while trying to look for someone. I can't say I've ever witnessed a case where these people were "taking up all the seats." Those seats at the gates are never comfortable anyway.
So my suggestion to this all (if the TSA wants to know), is for visitors to the airport who aren't flying, allow them access to the domestic terminals. But for these visitors, they can't bring any packages through the checkpoints. If they aren't carrying anything, they should be able to pass through security quickly. I don't see this as any more of a security problem allowing this. After all, has there been one instance of a hijacking of a commercial airliner by someone without a ticket? I would think someone bent on doing a hijacking wouldn't skip the most easy part (buying a ticket to board the plane).
My two cents worth.