Professor Jennifer Hurley writes about her struggles with rubrics, and why she's ditched them (emphasis mine):

The real reason I think we’re attracted to rubrics is because we like the simplicity and efficiency of them. Wouldn’t it be nice if we could make giving feedback into a scientific process? That would be so efficient and so less fraught with uncertainty! But why are we so afraid to admit that the evaluation of writing is subjective and depends much upon the individual reader? I think it comes back to grades. We’re frightened that if we admit that good writing is sometimes a matter of opinion, then we are no longer the authority, and we no longer can defend our grades.

I've struggled with rubrics myself, for exactly this reason–good work, especially complex work like writing or a presentation, is impossible to quantify into any manageable number of categories. Too often, I think in my head, "This is a B+ paper," and then I look to reconcile the rubric with that grade. Yes, the rubric does sometimes keep me on track while grading, making sure I look at each different area and don't forget to give credit where it's due. When the rubric isn't tied directly to the grade, it's slightly more useful, but only slightly.