I like the new rule. It means an enemy falling to zero hitpoints is "defeated" rather than simply killed. It opens up a wider variety of possibilities for the outcome of a fight, so you no longer have to assume that every fight is a fight to the death. The old system was flawed because it gave a mechanical incentive for the heroes to be murderous rather than merciful. All it really did was add to the general problem of D&D "Lawful Good reckless murderers," something that I have always despised.
Anyways, the only two arguments I have seen for the "non-lethal at -4 penalty" are based on either simulationist arguments, or a desire to have D&D be a "grim and gritty" game where it is more difficult to be heroic and battles are lethal. I don't think either argument works.
The Simulationist argument fails simple because it exaggerates the added difficulty of subduing an opponent. Certainly it must be difficult to hold back on a strike so that it is not a lethal blow, but that added difficulty is probably absolutely trivial to the kind of truly remarkable warrior called a D&D PC. A high-level swordmaster PC is a far better swordsman than anyone ever seen in history. Someone who can defeat ancient monstrosities like the Tarrasque with a sword, or even lesser threats like hordes of raging orcs, should have no problem whatsoever going for a knock-out blow.
The "grim and gritty" argument (essentially the one Celebrim is making) falters simply because it is a matter of preference. I prefer a cinematic style where death isn't so certain and the hero always saves the day. Since the game as a whole is moving towards the style prefer and away from being grim and gritty, I see no reason for this particular issue to be any different.
