Some parts of Piwik (tests and other non-production parts) require dependencies from non-composer package manages (like karma which is obtained via npm). We can't directly get them through composer because the dependencies do not have composer.json files.

HOWEVER, we can define the package info from within our composer.json file and load the dependencies that way. Using a single package manager will make some aspects of development simpler, and since it's possible, I say we do it.