It would be nice to be able to brew install homebrew/php/arcanist, for example, instead of git clone-ing two repositories "somewhere/" and extending PATH.

Unfortunately installing arcanist through brew installs a very old version which does not work with recent Phabricators. To fix this brew needs the weekly arcanist releases to be tagged, see https://github.com/Homebrew/homebrew-php/pull/3864#issuecomment-268447510

Additionally, if releases are tagged, it would be possible for users (of all platforms) who need to use an older Phabricator server to use a specific compatible version of arcanist.