Metaphor is a practice that gives a lot of people trouble. Perhaps it's because that coming up with a good system metaphor is hard, and requires creativity, not just work.

A metaphor is more than just a common vocabulary, though that's certainly a powerful benefit. And, in terms of common vocabulary, a good metaphor provides benefits that the domain vocabulary doesn't.

The domain vocabulary often contains subtleties known by the Customer that are not fully understood by the developers. The metaphor should be neutral ground.

The domain vocabulary describes how the business works. The metaphor should describe how the system works. In this way the two can work together synergistically. The domain vocabulary tends to describe what should be done and the metaphor describe how it is being done.

Many projects muddle through without a metaphor, but I would caution against giving up on it too easily.