What exactly does not work without this line? Do you get any errors?

I get exceptions from the Input.php class that a required argument is missing. It also differs from this doc that seems to have the correct example: http://symfony.com/doc/current/components/console/introduction.html#testing-commands

   $commandTester->execute(
        array('command' => $command->getName(), 'name' => 'Fabien')
    );

It was removed quite recently in 16e5577

If I remember correctly, you get indeed errors when you are missing the command option.

Let's put the 2 removed lines (see commit) back!

The problem for me is that I can't reproduce this errors, not even on a fresh 2.3 installation.

I created a gist: https://gist.github.com/WouterJ/6129648

Without command it produces a RuntimeException: Not enough arguments in Symfony\Component\Console\Input\Input on line 76

This seems to be a bug. If you create a Command that has arguments, everything is fine. Additionally, it even doesn't matter which value you pass for the command argument.

Was about to open an issue for this problem then found this. Bumped also the other issue for the bug.

@gena01 can you confirm that this problem has been fixed with symfony/symfony@50c8050?

Actually, symfony/symfony@50c8050 won't be merged into 2.2 or 2.3, but will only be a new feature in 2.4. Thus, we need to add the removed lines again.