It's really not intended as an argument against open source. It's an argument against using open source forum software (maybe even open source web app software

It's really not intended as an argument against open source. It's an argument against using open source forum software (maybe even open source web app software with user accounts). There are always exploits available and you can't always patch in time. Sometimes there isn't even a patch available before the exploits are in use. I was always taught never to rely on security through obscurity. I understand that it's a best practice to not. But feeding off of what jvdongen said, you are far less likely to be hacked using well-constructed homebrew software than an off-the-shelf open source solution.