Yes I know this has been discussed already :) but I'd like to start the discussion again with a nice summary of the arguments that were expressed:

@fabpot in #5748: For me, PHPunit is a tool and I'm not comfortable (yet) about adding this tool dep into the composer file.

PHPUnit is not just a tool, it's also a framework: we use PHPUnit classes to write the tests. So it's not just about downloading and running a phar, it's also about installing the code so that we can use it like any framework/library.

As a more concrete example, PHPUnit 4.5 has added important new features to the test framework and only by requiring it in Composer can you make sure people install the latest version.

@stof in #12136: Adding phpunit as a dev requirement means that each Travis build has to install 15 more composer packages (phpunit is not a small dependency)

@pborreli mentions this in #5748: "indeed, build takes 1 second more approx, which is not much"

Another reason I've read:

When working with VMs the global include path is not accessible on the host machine

For me autocompletion & co is completely broken because the IDE can't find the PHPUnit framework classes. It's much harder to work with tests. And by adding the global tools (i.e. global composer install) to PhpStorm I now have a lot of duplicated classes, which trips the IDE even more.

So hopefully we can open this discussion again and add PHPUnit to require-dev in composer.json :)?