in Aikido - and also in other Budo arts - are sequences of techniques in the way that only the last of the techniques ends in a final throw or a fixation. According to this definition, tsuzukiwaza are not concatenations. The concatenations consist of standard techniques of Aikido. The roles of Uke and Nage can change. Kaeshiwaza are counter techniques, so the roles change. If the roles remain the same it is renzokuwaza, which means after a "failed" technique Nage performs an alternative technique.
Nage starts a technique, Uke can block it and then begin a technique him/herself. What could be the purpose of kaeshiwaza?
1. Show weaknesses in the execution of techniques.
2. Being able to win against an aikidoka too.
3. Dissolve the fixed roles of uke and nage.
You don't have to answer weakness in the execution with counter techniques, you can teach it differently, as Tohei Sensei did it. Since an aikidoka is by definition peaceful, there is no need to defend oneself against him/her. Remains point 3. Usually, the roles in Aikido practise change constantly, so it is unnecessary to do this in a concatenation. Kaeshiwaza tends to weaken the quality of actions. Therefore, kaeshiwaza may promote agility or adaptability or blending with the other, but it will not develop the ability to clearly control a situation. Yoshigasaki Sensei has always spoken out strongly against Kaeshiwaza.