Why do a technique at all?
First, I always try to remember that we're training principles, and not techniques. We combine principles into techniques, and sometimes spontaniously create new techniques, but without the principles, there's no technique.
Having said that, I'm going to comment on the the statement above regarding an uke who clamps down with a stiff arm and is extremely stable. My initial response is why do any technique at all? Imagine a two handed wrist grab where uke just latches on and grips for all he's worth. Where's the attack? He's stable, and he's got a hold of me, but he's not doing anything to my center.
Sure, I could pop him in the nose and start my technique (and that may actually be a very useful way of dealing with that type of situation), but then I become the attacker. Alternately, I could focus on ki and body position and pull of a technique anyway -- if I'm good enough to do so. Now, if he tries to move me, then we've got a technique. Or, if he's just holding onto me so a buddy of his could attck me, then I'll move, probably pop him in the nose, and start a technque.
But, if he's not giving me any energy, then I really don't have anything to work with and actually (depending on the situation) don't have any reason to try to force him into a technique. Yes, atemi can be very useful, but we need to be sensitive to the situation and uke's energy. And anyway, the more energy uke gives us, the more effective our atemi will be. :-)
As uke, we're taught not to block in my dojo. But, that doesn't mean we willingly fly into atemi either! We strive to maintain the attack, but will move the atemi's target out of the way before we get hit. For example, I attack with a straight punch to nage's midsection. He puts out a fist in the way of my face while stepping off line. If I'm going full speed, I've got two choices: move my head back, or get hit. There's no way for me to block while striking if I'm going full speed. Sometimes uke will put up a hand because they know where the atemi is coming from, but then all nage has to do is switch where he applies the atemi.
This gets back to my initial point about training principles. I'm finding that so much of successful Aikido lies in good timing. When we train, we have to keep in mind that we're taking an encouter that can last as little as a portion of a second ("the space betwen breaths") and stretching it out over several seconds. However, no matter how slow we make the encounter, we should still behave as though it is happening at full speed.
I'm not saying that blocking is wrong in every circumstance, but I believe (and I've been taught) that it is not possible to block at the same instant one is executing a direct, powerful attack. The most we can expect to do as sensitive attackers (funny phrase *g*) is to get out of the way of nage's atemi.
My 3.5 cents, :-)