The maintainers of packages are supposed to port their software to the relevant platform, that's their job, and yes sometimes this takes some work, whereas Homebrew seems to assume that ./configure; make; make install should flawlessly work out of the box for pretty much anything (this is not true). The fact that they actively discourage even trying to install outside of /usr/local is simply laziness (and not in a good way). This is a foolish and naive approach, and pursuing this idealistic, reductive approach for "simplicitly" breaks down very, very quickly. MacPorts is a well behaved, self-contained system, and a well designed one at that. Homebrew just dumps everything into a system directory that it really shouldn't have exclusive rights to in the first place (/usr/local).
Moreover, there is nothing wrong with installing extra compilers, libraries, etc, under /opt/local and having them run alongside the system libraries. Lots of systems have provisions for doing something like this.
Also, using version control systems as distribution and deployment systems is a horrible anti-pattern and I really, really wish people would stop doing this.
Homebrew's documentation is terrible. Its terminology is facile and stupid in the way it analogizes everything with beer.