Personally I hope to achieve such a high level of proficiency that atemi would not be necessary. That's somewhere down the road. For now though, when I practice I probably use it more mentally than physically. It helps to remind me to activate both sides of my body and keeps me centered. But I also think (physically) atemi has benefits for both nage and uke. It gives them both something to think about.
Bruce I can't say that in my experience that almost all irimi movements require atemi. I think that may be more of a stylistic option, although you can execute atemi at various points throughout a technique. If you study any technique I think it is safe to say that atemi is usually a backup for poor timing, spacing, or some other issue. It's always there as an option but like I said, I hope to attain such a high level of expertise that it would not be necessary.