The day when Slackware picks up systemd is probably when I throw in the towel and just switch to MacOSX or FreeBSD.
Seriously though, I would like to know what is unusable out of the box in Slackware? Granted, I appreciate it's not the most new-user friendly, but I wouldn't consider it unusable. In my opinion it is the best option for those that just want "plain ol' Linux", and know or want to learn the nuts and bolts about linux (it is easy to understand the entire boot process and read every line of the init scripts - it is not bloated). I also appreciate the simple package management system, combined with slackbuilds.org, which makes downloading/upgrading custom packages as simple as using tar and editing a single build script. Slackware also handles 32-bit and 64-bit very cleanly using AlienBob's stuff.
Having been a Linux user since 1994, I still prefer Slackware because I know how it works, how it boots, how to build packages, and it's the about the closest thing to vanilla kernel and vanilla packages as you can get and still have the advantages of a distro.