I think we may be talking about two different but related things. I have trained in a striking art, and I've been in a couple of real world fights. I agree that a good attacker will leave few openings (if any). But that's different than actively blocking during the instant
of the attack.
If I'm in the act of throwing a powerful, centered punch, what would I block? How could I block? After all, all of my energy is directed into that punch (actually into and through my opponent). If I see something coming toward my face/midsection/groin, the most I can do is move. If I try to block, then I've switched from attacking to defending, and my attack will suffer -- it will no longer have my full commitment. In fact, even moving breaks my structure and makes my attack less powerful. That's one reason why atemi are so effective at breaking uke's focus.
Remember, I am speaking of the instant of the attack. As soon as that instant passes, I can defend, attack again, or retreat depending on the other guy's actions. In fact, I'd suggest that really effective martial artists who study striking arts are so good at switching between attacking and defending from one instant to the next
that it seems they are doing both at the same time. For example: an effective block followed immediately by a fully committed, centered attack could very well be one motion.
And, heck, I'm a beginner, so I'm sure there's a lot about the subject that I don't know. However for me, at this stage in my training, I find it helpful to strive for every attack to be like the one sword cut: no thought for myself, just pure focused attack.
Of course, I also remember the other part of ukemi (safe falling) so that my one, good, completely focused, and committed attack isn't my last.