You completely misunderstand the point. "Another package manager" doesn't matter. People can and should use the package manager of their choice (why should I break pip?). The point is to have common ground between all package managers.
Could you talk about the advantage(s) you perceive of a "common ground between all package managers"? After using npm a bit I think scripting hooks in a package manager that can call other package managers enables many good things.
Well, for one thing, new package managers could be implemented without having to worry about the infrastructure. The server software could be shared between all linux distributions and would be applicable for any sort of "bundle server"; think game addons, firefox/chrome extensions, skins for your favourite software and what not.
One is that you can easily specify dependencies on packages not written in the language your package is written in, in a way that's machine-readable. Imagine if I could do "npm install some-framework" and it'd automatically build and install the relevant versions of node.js and MongoDB as well.
