I absolutely hate that this is modus operandi for the homebrew and ruby communities. Who though that this is a good idea?
I'm not validating it as a good practice, but that seems to be the main reason and I've not heard of Pow or Homebrew causing any problems in this regard.. yet ;-)
There's a reason I don't install things that don't use apt.
There is a manual installation process provided on the front page; Yeoman itself appears to be a Node.js module in the node package manager npm.