I have been both on the giver and reciever of unsolicated kaeshi waza. I can remember one occasion when I was very new to Aikido, it became a Valuable lesson to the correct form of irimi-nage. I stepped too soon leaving my hand a little to far back. Since I was practicing with a judoka, let's just say I learnt my lesson with a nice hard Judo style koshi-nage.
I do see a need to perform this kind of practice but it should only be on occasion. If you are focusing too much on kaeshi-waza then the lessons the sensei is teaching might be missed, not to mention that kaeshi-waza is not about Nage making a mistake. It is about Uke's skill to use a technique and become Nage. In that case the practice is much more different. Whenever someone is performing the role of Uke they should always be practicing this principle, they don't have to completely reverse the technique, but know that they have generated a situation where they could reverse.