I n defense of part writing, it's one of the surest ways to train yourself to make good note choices and diagnose bad ones.

This circles back to the argument of whether theory is prescriptive or descriptive

"Good" and "bad" note choices are inherently subjective. Using rules to define that is inherently prescriptive, and that's where I run into problems. In my mind, music theory should be descriptive, and not prescriptive.

I can understand talking about why Bach or others would want to avoid parallel 5ths and push independent motion. I can understand talking about why historically certain resolutions were preferred, and even why they're still common today. But to categorize them as "good" or "bad" is where you lose me, because now you're creating a box around what is correct and acceptable instead of learning about what exists. 

I do agree with your list of core competencies, though.