You don't make a very compelling argument at all for me to switch to Gentoo from Slackware. Compiling binaries sucks ass.
For the bigger packages, like openoffice and firefox, there are -bin versions available which simply download and install the binaries.
It's possible for people to make ebuilds for anything, whether it's sourcecode, pre-compiled apps, commercial closed-source apps, etc. emerge doesn't *have* to compile stuff, and, in some cases, doesn't.
I'm only doing openoffice from source because I'm curious how long it'd take, and because it's fun to try