There are certainly a lot of long posts here. While I read through each one, it is also possible that I missed some points and am being somewhat redundant.
Thus far, the underlying current seems to be a discussion of how much force is necessary, and whether or not you should hurt an attacker.
I believe we learn ukemi for a couple of reasons, two of which I will mention here. One is to allow others to practice their techniques (absolutely necessary). The other is to be able to take techniques without being seriously injured every time.
This is where the pain an injury part of practical 'real world' Aikido comes into play. While Aikido practitioners do not necessarily want to hurt an attacker, it would really be fairly difficult not to do so at least a little. An attacker is unlikely to have any idea how they might best respond to a technique, and thus end up spraining or breaking something during a move that would have only pained a partner in training. This wouldn't be because they were necessarily stupid (though this is up for debate if they are going around attacking people), but because they probably won't realize just how in control of them you can be. They probably won't hit the ground gracefully or turn their body just right, etc.
I'm probably not the only person who has had a technique hurt on me a little more than it had to because I inadvertently tensed or moved in a bad way. The effect could be greater on a person with no familiarity, and their reaction is difficult if not impossible to predict.
With control, you can somewhat determine the outcome. However, you do not want to do 'too little' or the person really won't stop (if you've seen or been in a fight (or seen COPS enough times) - which usually involve drunk people - you'll know this is largely true). It's a physical confrontation, and frankly, a physical attack needs an appropriate physical response (you're speaking Combat, and a person well-trained in Aikido is an excellent linguist). This is why I agree with the multiple iterations of the fact that the energy into an attack should, and typically does, translate into the energy you transfer through the technique. You are not really blending if it happens another way (powerful technique on a weak attack takes additional energy, just as slowing or weakening the technique does on a fierce attack). A person attacking softly will be naturally put down softly; an attacker going for the gold, so to speak, should receive a firm smack down.
So, evolving to a point: Somebody in a physical confrontation is going to be hurt in one way or another, so as long as you're in the right, it best not be you. It's just important that when it's done, you let it be done. That is partially how I read into the "don't want to hurt people" ideal.
In fact, it's pretty much illegal to hurt people excessively, and there have been martial artists from many disciplines who have had to face charges for "defending" themselves (I knew this kung fu guy who got into some trouble when he got a litte over-zealous one time).
As with many, I agree with ending the confrontation on a personal level (attacker against you) and then calling the police to take care of the criminal and social aspects.
I rather hope this makes sense, as I don't intend to do a solid proof-read before posting like I should.