I've done this exact thing very recently.
I compiled openssl-1.0.something from source on a Slackware 11.0 linux system. I wanted to patch the login-by-password thing to pause for 7 seconds on a failed login, so as to tarpit those annoying Chinese ssh password guessers.
I compiled Apache httpd 2.2.17 on the same Slackware 11.0 system because it came with Apache 2.0.something, and I wanted to use a 2.2.x httpd.conf from another system.
Come to think of it, I also compiled Alpine 2.0 (email client, pine follow-on) for the same system. Why Slackware 11.0? It's running on a circa 2003 machine with Intel "Brookdale" graphics, and any later Slackware just doesn't support the poorly-documented Intel graphics.
On a different machine, I pulled the TI ACX111 wireless chip support from github and compiled a new driver, allowing me to use a $9 wireless card.
I also habitually recompile Linux kernels to make them specific to the machine they run on.
So in my case it's various reasons of customization, hardware support, and "just because".
Given what we know of epidemiology in a uniform population (http://www.usenix.org/publications/login/2005-12/openpdfs/geer.pdf) why doesn't everyone compile from source on their own systems, with whatever compile-time options they choose? That would make for a vastly more virus- and worm-resistant population.