I agree and generally teach back break falls while including kicks. However, a truly affected leg lift will take you on a long trip. Even very advanced students will have a very difficult time taking full falls with one of their legs held up against their chest. It is also dangerous because if someone panics on an accidentally extra big back breakfall they are likely to forget their head or brace with their hands, either one leading to a world of pain.
movement vs. kicks, and the general sense of disbalance is taught. I then allow them to throw me. (I teach four very beginners, so that is more than enough.)
Regardless, I believe that kicks of different varieties should be incorporated more into aikido study. I also agree that if you aren't used to a kick, you won't see it coming, and won't be able to react, no matter how much you know about fists and grabs. Feet move faster!