(1) 'Blood sport' approaches are degenerations of adversarial method.
(2) Adversarial method is one of the best methods of philosophy.
His argument is interesting on both accounts. I think (1) is fairly clearly right; and one can in part see it by the fact that criticisms of bloodsport are often rejected on the basis that adversarial method or debate plays a key role in philosophical discussion -- that is, bloodsport gets a pass because it can masquerade as debate and argument. I'm much less inclined to agree with (2), though; I think there is a genuine place and a genuine value for adversarial method as Kenny understands it, but it is a secondary place and a limited value.
Kenny gives an analogy with adversarial method as found in law:
The adversarial method is best known from its use in law. The theory (which is at least as degenerate in law as it is in philosophy) is that if equally matched opponents argue opposite sides of a matter, the side of the truth will be at an advantage. For this reason, no matter how obvious a person's guilt seems, her trial is not judged to be valid unless she has adequate representation. She must have an attorney who has done a competent job of defending her.
Obviously this can only be analogy. For one thing, in the case of law you are not really so much interested in truth -- well, at least not immediately and directly. Your primary concern is not discovery of truth but adjudication, i.e., reasonable and orderly resolution of the dispute; and adversarial systems come into play when the sort of adjudication you want is the sort in which both sides should be given full rein (or as close to it as is genuinely feasible and reasonable) to present their side in what they deem to be the best light. Truth acts as a constraint on what's reasonably allowed, but it is not the primary interest. In matters where it isn't in everyone's interest to be allowed to do whatever it takes to present their side well, another method of resolving the dispute would have to be used. For instance, the usual view is that in matters of family law adversarial method is often contrary to everyone's interest; and, in our system at least, family law tends to be handled by collaborative approach, in which everyone agrees to reach an agreement if there is any reasonable way to do so, the adversarial approach being a last resort if the collaborative approach breaks down. In many systems, you have a so-called inquisitorial approach, where the two parties are relatively constrained in what they are allowed to do, and many of the burdens and responsibilities we place on the adversaries are placed instead on the adjudicating party.
I dwell on this somewhat because third-party judgment is a key difference between adversarial method in law and the sort of adversarial method Kenny has in mind in philosophy. We get a lovely illustration of this in Plato's Gorgias. Socrates and Polus, a Sophist, are arguing in front of an audience, and Polus keeps trying to play to the crowd. Socrates points out, however, that in the sort of argument they are having the only jury or judge that matters is the person with whom you are arguing. Thus in philosophical matters, the adjudicating party is not a third-party; in most cases, each of the two parties to the discussion has that responsibility. (I say 'in most cases' because in certain types of pedagogical discussion, e.g., a medieval disputation, the responsibility will fall chiefly on the teacher. Even there, however, there is a sense in which it falls on the students as well.) I think Kenny would agree with this, at least more or less, because I think it is this that is responsible for a key feature of adversarial philosophy as he understands it:
How do we ensure that the two sides are equally matched? Well, when your "opponent" is struggling to find a good argument for his position (or against yours), you have to help him out. I think (or at least I hope) philosophy professors do this for their undergrads all the time. We have to get the strongest form of the position, and the strongest arguments for it, in order to be able to evaluate whether our attacks against it succeed; otherwise, we are in danger of the strawman fallacy.
In law the relatively equal matching of the two sides is established by neutral procedural rules; but this is not operative here. If one side is struggling, and the other side is merely taking advantage of this weakness rather than allowing the fair consideration of arguments, the opportunistic side is not violating any procedures of disclosure, or anything like that. Rather, it is contrary to the spirit in which the discussion is supposed to be held: as a philosophical discussion, it has certain ends -- understanding, truth, wisdom, etc. -- and such a means will tend to be poorly proportioned to that end.
But I think in most cases, in fact, there will be better means to those ends than adversarial method, as Kenny understands it, can provide. Not every philosophical discussion is a dispute between sides; most, arguably, are not. Nor, even in a dispute between two sides is it always necessary to take an adversarial approach; there are approaches that forego ordinary adversarial opposition in favor of collaboration or investigation. I'm inclined to think, in fact, that the best place for such methods -- the place in which it can genuinely be considered one of the best methods available to philosophy -- is in arguing with sophists who can be argued with. (Plato's Gorgias again. Of course, if you are faced with having to argue with sophists who can't be argued with, you may well have no good method available save, in Boethius's phrase, victorious death.) You'll still need to raise the strongest points; but this can be done in circumstances that involve no opposition or adversarial character. I think philosophical blogging is often a good example of this, since I think that, contrary to Kenny's suggestion, it is usually investigative, not adversarial. On this issue, for instance, Kenny and I are in no direct opposition; we are simply conducting contrasting investigations into the same topic. These can then be compared and contrasted and critically examined in future investigations; but there is nothing really adversarial about it, and it really doesn't matter whether either of us is giving the strongest argument available.