>[Ruby] is not best for performance, installation or deployment.

PHP isn't either, except that it's packaged well in Linux.

If there were good packages for Ruby in Debian, the only advantage that PHP would have is in being the shared hosting default. Both are pretty equally performant, and Ruby has the advantage of being a nicely designed language.

>there is absolutely nothing to be gained by not using PHP.

What is there to be gained by using PHP?