I think there is a reason for it... ukemi (getting thrown) can be a tool for developing connection and structure, whilst at the same time it can be a form of body conditioning. It depends on the purpose and goals to which one is training for.
Personally I enjoy the physical workout from doing lots of "getting thrown". But these days, it is more of a choice of whether I take the fall or not. If my student hasn't got my balance and even if they do, I have a multitude of choices available to me, including reversing it, stopping it, remaining immovable, hitting them, or simply falling over. It all depends on what my goal happens to be at the time - and what I want to work on OR what I want the student to work on. And even if I do take the fall, my legs are still viable and there is always an opportunity for taking it to the ground, even as I am falling...
The problem with kaeshi-waza, though, is that it too demands that tori become uke and receive full ukemi and be thrown!
I don't think it's necessary to be thrown... ukemi means to "receive" with the body. Receiving what? That IS the point. Not to receive a throw.... but to the point where if you do get thrown it's only because you didn't feel it coming, until the last moment when you find yourself being "pulled up"... right before you hit the mat.
As for kaeshi-waza
being more "advanced"... that's also dependent on who's teaching and what they are teaching. My guys get to do this as soon as they know how to take a decent breakfall.
Ever notice at seminars that the "higher" ranking folks don't ever let you put a technique on them? Notice how they "tank" before you get to put the tech on? Hmmmm..... I wonder why that is...
The worst part is that people learn by EXAMPLE.