I also have no problem with any behavior that doesn't involve hard-coding opponents to go after. As far as bringing down scores of top bots, a few thoughts. First, I'm skeptical of how much it will bring their scores down, for the reasons Wompi said. I'm also skeptical it would bring any of them down more than the others, which is what really matters. And even if it does bring some down more than others, that just means there are strategies to deal with the Grudge strategy that are better than others, which is fine and totally legit.
I get your point. The only robots that won't be effected by it are the ones who don't fire at it. Actually the best way to deal with grudge is to put another robot between you and it. It isn't intelligent enough to go around other robots to get to its target. So it will ram and fire right through some other robot.