I think an interesting side-by-side test would be to inject one brisket as many do, and then jaccard another brisket and marinate it. Cook the same and compare. Leave all the other variables the same as well of course.
Sounds like an experiment I might have to try myself!
My hypothesis, were I using the scientific method and all, is that the jaccarded brisket will be more overall tender, but less juicy and maybe even less flavorful.
Where I personally use the jaccard is on leaner meats cooked at high temp and not to as high an internal temp. I think any cut of meat that is going to be cooked quasi low n slow to an internal temp where the meat becomes tender will benefit little from the mechnical nature of a jaccard's tenderizing.
We did a flank steak for fajitas the other night for dinner and I jaccarded the daylights out of it before marinating. Turned out most excellent.