One, turning a weapon against itself is a poetic concept. Two, it's a technical challenge that lets ethical programmers share in the fun of designing viruses. You don't have to convince people to install patches and system updates. You just use the technology to force them to do what you want. Therein lies the problem.
Beneficial viruses are a simple solution that's always wrong.
Viral propagation mechanisms are inherently bad, and giving them beneficial payloads doesn't help. A virus isn't a tool for any rational network administrator, regardless of intent. Installation is adapted to the host. It's easy to stop an installation in progress, or uninstall the software. It's easy to know what has been installed where.