It doesn't really. In theory more eyes on the code means more chance for a security bug to be found, either by white hat researchers or black hat exploiters. In practice this doesn't really pan out; not only are most free software projects small hobbyist endeavors, but even large free software projects with many eyes on them, such as OpenSSL and curl, have had critical security vulnerabilities over the years. When it comes to security issues, having the right eyes on the code matters more than having many eyes.

The original promise of free software, the four freedoms, is all it guarantees. In my opinion this is enough to prefer free software over proprietary.