If you want to spend 6 hours debugging a problem when one eventually crops up, because you have neither any idea how the underlying configuration works nor which layer of the config tools built on top of that to look at.
Besides, what do you mean by "installing"? The initial system install? That's something you do once, and given Gentoo or Debian, probably never again (as you can seamlessly up/downgrade at will). If you mean software in general, thpfffft. emerge program-name is all it usually takes on Gentoo - and a bit of waiting possibly. On Debian it's apt-get install program-name and you're done.
Slackware is much less comfortable, but when I'm setting up a server I know what I want, and what I certainly do not want is pretty GUI tools (or an entire X server and all of its stability problems for that matter) on the box. I certainly don't have fancy graphics cards, fancy "multimedia devices" or anything of that sort to justify a clickety cute installer with lots of doodads.
Have you had a look at Knoppix, perchance? It's a full fledged Debian live filesystem that boots off of CD and detects your hardware automatically, and with great success according to most everyone I heard talking about it. Comes with a KDE3 desktop (if you want such a thing), OpenOffice, Mozilla and lots of other stuff (they squeezed over 1GB uncompressed onto the ISO). And you can install it on HD and have an actual Debian install ready. Doesn't get easier than that, even with RedHat.
Not to mention these distros are not full of bloated scripts that bog down the machine and spiked with repair-myself-automatically mechanisms that just get in your way.
Makeshifts last the longest.