The problem, however, is that no other Linux distribution looks like Slackware anymore. Everyone has moved to Sys V init first and then some to Upstart, systemd, and others. Other distributions have sophisticated package managers and source package formats. Other distributions use PAM for authentication.
So, effectively, when you learn Slackware, you learn Slackware. If you have to use Linux in the real world, you might as well learn a widely-used distribution such as a Debian derivative. Or Red Hat if you like suits.
Everyone had moved to sysvinit years ago. The point is not that Slackware did what the other distros did, minus the handholding utilities. The point is that Slackware requires you to figure it out yourself, read documentation, go to the IRC channel, and grep around /etc until you got something working the first time, a skill that will help you with any flavour of Linux--no matter which init system it uses.
The problem, however, is that no other Linux distribution looks like Slackware anymore. Everyone has moved to Sys V init first and then some to Upstart, systemd, and others. Other distributions have sophisticated package managers and source package formats. Other distributions use PAM for authentication.
So, effectively, when you learn Slackware, you learn Slackware. If you have to use Linux in the real world, you might as well learn a widely-used distribution such as a Debian derivative. Or Red Hat if you like suits.