[mercer]

Actually, I'd say it's the other way around. Don't use a task runner because in many cases the operating system and the tools themselves already support what you need: parallel and sequential execution, grouping a complex set of commands under one 'name', etc.

Where your analogy does make some sense though, is cross-platform support. In the same way that jQuery solves cross-browser issues, a task runner solves the problem where your npm script commands might not work on non-*nix systems, or vice-versa. But that's a bit like using jQuery exclusively to solve XHR incompatibilities, instead of a library like superagent.