* In the closed-source world, you have a bunch of black-hats trying to find exploits in the software, sometimes by reverse engineering the product and sometimes using source code that has been accidentally leaked. Opposing them are a limited number of white-hats that work for the closed-source company that controls the software. As an administrator, you’re dependent on the technical talent of the closed-source company, it’s budgets, and finally the politics of its PR decisions (see CiscoBlackHat2005) as to whether bugs are found, fixed, and reported to you.
* In the open-source world, you have a bunch of black-hats trying to find exploits in the software using the source-code itself. In contrast to the closed-source world, however, you also have an almost unlimited number of white-hats doing the same thing. Those white hats are not limited by the technical talent, budgets, or politics of a single company. As a result, more bugs are found, fixed, and reported more quickly. You get more information, faster, in order to keep your network secure.