What is a non-tenure-track assistant professor position in the USA and is it harmful? I see some offers on the Chronicle jobs website (such as this one from UC San Diego) for non-tenure-track assistant professor positions.

Is the work of a non-tenure-track assistant professor exactly the same as the work of a tenure-track assistant professor, e.g. in terms of teaching/research ratio?

Also, would taking on a position of this type be more, less or equally helpful compared to, say, taking up another postdoc, for someone that would later want to apply to a tenure-track position?

Forgive me if this question does not make sense to you, but I have taken the impression from some sources that whatever professional decision one takes that is slightly outside the "normal" progression of an academic career can be harmful.
Actually, in this specific case I think it may just be a mistake in the ad.

The application page linked from the Chronicle ad just describes the position as "Assistant Professor" and doesn't say anything about tenure one way or the other.  Certainly the default for an assistant professor position would be that it is tenure track.

Also, I attended UC San Diego as a grad student (in a different department) and don't ever remember hearing about any non-tenure-track assistant professors.  Full-time teaching-oriented faculty at UCSD have the title  "Lecturer".

So I think there's a good chance that these are ordinary tenure-track assistant professor positions, and that someone just clicked the wrong box when submitting the Chronicle ad.  

I would suggest getting in touch with the department for clarification.