Indeed. You make a very good point there. A genuine contributor would not care that much about his posts/activity not being counted; he would care more about the discussion itself. I believe that including off-topic will even significantly reduce the spam/account farming that has been going on there.
You guys are optimistic. I foresee there would still be the same amount of spamming, except now there would also be a lot of people asking why their activity is not increasing.