Some practices are better off _not_ raised to the level of principle. Jury nullification is one such perhaps.
Aside from whatever _should_ be juries always have in practice (though there is no way of determining how often) exercised "jury nullification" more or less spontaneously without necessarily ever having heard of the phrase iself or particularly theorizing their action. And of course white juries in the south trying KKK members etc. are the most famous instance of this as a regular practice. And I would say that [objectively?] juries trying cops regularly exercise jury nullification -- they assume the law says cops can do anything, particularly if the victim is black.
Also -- I believe a sort of jury nullification has in the past occasionally have been the route by which the populace has changed the law. This is a vague memory from over 50 years ago, but I think I remember reading that in the 19th century the British Parliament changed the law on attempted assassination of the monarch because there had been a fairly large number of cases in which juries simply refused to convict obvious mental cases.
A tentative prediction: the only (relatively peaceful) way policed brutality can be brought under control is by large-scale "jury nullification" on a random basis in cities where the police are too obstreperous. That can't happen under present political conditions -- but it is more likely than that courts, legislatures, or mayors doing it. (Extravagantly put: if the odds are one-billion to one against every course of action but one, and the odds against that are only 900 million to one, the latter is clearly the more realistic course of action.)
Carrol