"mock" is a pretty cool tool to build cleanly with. I use it
extensively. But it's a *huge* burden of compilation time and CPU
cycles for building new packages, especially if you lack the resources
to configure it for and maintain an internal CentOS or Scientific
Linux mirror to build your chroot cages from.

I'm often saddened, but not shocked, to find that people porting
packages haven't bothered using it.