Should I decline authorship over the quality of a paper?
So long as there's nothing wrong with the quality of the work, I'd let the person in the field make the decision about it's worthiness for publication.  Often, what's humdrum in one field is new and exciting in another.  I've seen people making livings moving routine info from one area to another.

When seeking collaborators in the future, my opinion is that having published a solid, if not exciting, piece of work in the field will be more advantageous than having published nothing.