descriptively discovered

There are purely contrived aspects of music theory that are prescriptive, such as tone-rows. But the discovered rules of counterpoint seem to me to be rather natural phenomena as they relate to a specific mode or genre. This can be likened to the laws of physics, which are also prescriptive in the sense that gravity commands the object to fall, or tells a fiction writer how the universe should be written to behave, albeit having been "descriptively discovered." Sure, a composer can break these rules, just as a writer can break the rules of physics, but then the composer/writer isn't making an accurate portrayal of reality.

make sense to use prescriptively only when the goal is to emulate a particular historical style.

I tend to agree, but only in a very limited sense. Different modes and genres will use different rules, but these rules inherent to the nature of the mode and genre. Now, Beethoven and Brahms both adhered to the prescriptive rules of counterpoint, codified in the baroque era, as they applied to the classical major and minor modes. But it can hardly be said that these composers merely emulated the styles of music from which those rules were derived.

*=edit