I would argue that Slackware is for people who have a better understanding of how the system fits together than many (but certainly not all!) of the rpm/deb package users. I use Slackware in an Enterprise setting on 70+ servers that cover everything from email to web-hosting to firewalls to custom built "sales presentation" devices. For us, slackware gives us complete control over the systems, without having to guess at what other services or programs may muddle with different parts of the configuration. It's easy for us to disable and remove any services that are not necessary on a particular computer, and we have our own custom installation, testing, and deployment scripts that allow us to keep machines with similar purposes up to date and in sync. While we could accomplish the same things with virtually any distro, Slackware is (for us) the easiest to do these things with, and "Just Works".