+1. This eliminates a whole class of bugs in which you declared phpunit as a dev dependency but end up using a class that it brought in without declaring as a regular dependency. Without an external linter, you can’t really catch that until your production code doesn’t bring the class in and throws a fatal error.
2. If you use phpunit class in prod code, you deserve to get a fatal error
2. That doesn’t apply to PHPUnit specifically, but if you, for example, import PHP-cs-fixer as dev dependency, it will bring symfony/console, and if you rely on that on your own code without importing it on composer.json as a regular dependency, the class will be missing when you composer install for production.