People without tickets are no more a security threat than people with tickets. (As has already been mentioned, all the 9/11 hijackers had tickets. Anyone intent on mischief won't be stopped by having to buy one.) Therefore, keeping them out does not enhance security.

If admitting people without tickets increases gate area congestion (perhaps to the point of impeding response to a real security issue) or waiting times in the security lines, let's deal with those issues on their merits. I personally doubt that there will be enough non-ticketed visitors to make a significant difference in either regard, especially when we consider that many of those who want to be there are already buying refundable tickets or getting gate passes, but in any case let's deal with the real issue(s) honestly.

Calling the exclusion of non-ticketed people a "security" issue is part of the current syndrome of invoking security as a mantra any time an airline or other authority wants to impose a rule without having to explain it. Its overuse reduces its credibility on those few occasions where it actually is the reason for something and where an explanation actually might be a bad idea.