It isn't that "mixing bits" of Aikido somehow spoils the "brew." ...
As far as I was told, the main difference is the "spirit" of fighting. I was told that you can not develop the "true spirit" of KSR when practicing aikido. (Interestingly Tissier often talks about the spirit of the swordwork, derived from KSR by Inaba sensei, being exactly the thing that interested him so much.)
And that you can not reach a true undestanding of the philosophy or esoteric teaching of KSR.
I can't imagine that an aikidoka who enters a koryu thinks about changing the tradition of that koryu?
"what if I move like this instead of what I am supposed to do in the kata?" is simply not imaginable, isn't it? Not when doint KSR. Not when doing TSKSR. Not when doing any other koryu budo.
And I don't think that this is the point: Through my discussions with German members of the KSR I learned, that the decision of not allowing aikidoka to enter the ryu has very deep contentual reasons. They truely think that someone who practices aikido has made decisions - in his mind and his body - which don't allow him to use body and mind to do what has to be done or to think was has to be thought when doing KSR.
With regard to the understanding of kata in aikido: As far as I am concerned I learned an learn aikido through practicing kihon no kata. Precise movements, static in the most basic form, bearing the same feeling like doing kata in kenjutsu.
I can't imagine how it is possible to "elicit a certain quality of movement or sensitivity in the practitioner" if not by doing the precise kata over and over again.