Irimi - tenkan is not the best way to describe the possible variations of uke nagashi. Soto - uchi (outside - inside) would be better. There are many variations possible (usually defined by three levels shita-naka-ua), and there in lies the strength of uke nagashi, not so much in selecting one or two that are supposed te be more effective/realistic.
Uke nagashi is a much valued technique in other martial arts as well, from Shorinji Kempo to Wing Chun.
Thank you Tom. The principle of getting off centre line is built into Aiki-Jujutsu but we don't work so much on changing our attacker's line. It is the same as the principle of irimi, these are words not found in our AJJ vocabulary and if the principle is there it is a very subtle one. I like the way Aikido articulates and expresses these principles when judging maai and blending with your attacker's energy. I think I will try and incorporate uke nagashi irimi into my aiki variations for my next grading.