Its the argument that the source must be hidden or else people will release exploits for it. e.g. The fact that Linux is open source has caused people to release exploits for it.
With exploits, you can easily fix them. The problem with this sort of algorithm is that you have people directly competing, and there's no objective measure of site quality. The system has to use what it can measure, and this means it's gamable. Security through obscurity sucks, but it's better than having nothing at all.
