But applications are tools to do a job. When I use a hammer I hit a nail, job done. Hammer can be laid down.

I don't need my hammer to be frustrating to use, a nail-hitting-service subscription model, have social chat options... etc. You get the idea.

Like the idea of a good butler, the best tool is one that removes a concern for the user, and doesn't take any more attention after that.

I personally believe many of these metrics are negative indicators of UX quality. Session length, pages viewed, number of clicks could very well mean that users aren't finding / getting what they want from your app.