I've done some benchmarks with millions of rows, and a default PostgreSQL installed via apt-get on Ubuntu works fine. I doubt whether the majority of PostgreSQL users really need to install from source (especially considering how much harder it makes upgrades).
I didn't say it was faster, and I didn't say the majority needed to do it.
The reason is to be able to customize your build and/or apply patches. Tom Lane fixed a production bug I reported in hours and this got me running fairly quickly.
