Using package.json is simpler when you're working on multiple projects. It also means that if you decide to contribute to a random project, you can simply run

Using package.json is simpler when you're working on multiple projects. It also means that if you decide to contribute to a random project, you can simply run "npm test" to make sure your changes are valid. No need to screw around installing mocha or karma or whatever test framework the project is using. You can safely assume that everyone that's using node will have npm, so it makes sense to keep tasks contained in npm.