I guess I am about a month late for this thread, but when you only check aikiweb once every few months you are late to most things.
I found the daito-ryu video very interesting and definitely see a similarity in their kamae and that of the yoshinkan. The history of kihon dosa that Inoue Sensei spoke about relates to the 6 basic movements of Yoshinkan, but not necessarily, i think, to kamae. The two kamae are much too close for anyone in our style to claim ownership of it.
As to why we do kamae, it is a learning tool as much as anything. Kamae helps focus our body, relax the arms and shoulders and allow power to flow through the legs. One of the exercises we regularly do is to have someone push on the top arm - you have to keep the arm relaxed and absorb uke's power. when you do it correctly you feel the strength of uke's push flowing through to the back foot. The kamae practice also teaches you to maintain a good posture which allows the power of the legs and body to flow (instead of arm power).
As we get better at kamae it changes, and becomes less rigid. After the senshusei course they guys have strong but rigid kamae that, if they keep training, becomes more relaxed and smooth. My kamae is much different from when we practiced in your dojo many years ago.
The top instructors do not need to do basic kamae to have that power. You can see that in video of Mustard and Takeno in addition to the ones mentioned. They will start and end with it, but in a jiyu waza they will not always have their hands up in the basic kamae. But watch and you will see the feet, hips, back and weight are the same.