I have only used Kaeshi-waza when:
1. Nage was using excessive force and slapping the mat or thigh yielded no relief, and
2. When Nage continues to disrupt specific tecnique instructions with remarks like "How are you going to get out of this"
Number one occurs with a greater frequency and is something that no one should have to do. Number two Does not happen often because I simply explain not to deviate from instruction and that such questions should be held for after class.
Although I am capable of Kaeshi-waza, it was something that I had to figure out because in my years of training, I never saw it emphasized.