Originally Posted by acid_kewpie
There is no proper use for it ...
well, I know you're no fan of Slackware
Slackware uses runlevel 4 to run the X-server.
In fact the definition of runlevels depends on the distribution. With the highly configurable distributions (for example Gentoo) you'll more or less define your own runlevels.
As an example you can define a special runlevel for a laptop which is in battery-mode and adapt it for less energy-consumption.