Why? Why should the uke side be practicing something different from the nage side? Why would you spend half your mat time practicing something that isn't aikido?
Hang on. Isn't that the point of uke? Shouldn't uke be playing the role of an attacker in order to allow nage to practice their technique in a simulated attack scenario? If uke was all about blending and being 'aiki' all the time, then how would nage be able to practice effectively?