The term riai means, literally, a blending of [ movements of the mind]. By understanding Aikido through riai, one sees that the taijutsu techniques were developed from movements using the sword. Therefore training with the sword will develop taijutsu techniques.
The Founder said that a weapon should be used as an extension of your body [ – and of your ki-body, thus defining your ki-envel0pe]. However he stressed that one should not develop a dependence upon [actually having] a particular weapon. To build [this way of using your ki (kimochi)] one should practice the basic exercises of ken and jo suburi, tai no henko, and kokyu dosa consistently. A good understanding of these basic exercises will enable the practitioner to move smoothly and surely with or without weapons.
– – – Morihiro Saito Shihan (presumably), inside front flap of Traditional Aikido vol. 1