My 2 cents:
I know some companies running servers on Gentoo. However, I personally would not do it. My reasoning is that a server's main characteristic should be reliability. I've emerged some packages that break apps or refuse to install until a newer version comes along, and that is just not acceptable for a server.
If I emerge world, and my SMB, Apache or MySQL dies, I'm going to be on po'd guy.
Even though Slackware is not as easy to update, you can be certain that any package included in Current will have been tested thoroughly.
Besides, you can always set up a cron job to use swaret or slapt-get every day to sync with Current.