Re: Backward Roll - Risks and benefits
I agree with you that it is very dangerous. From my experience many Aikido practitioners don't pay enough attention to safety during practice. There are many potential risks for injuries, especially neck injuries. I think that all throws are dangerous and must done with great consideration to the uke's safety.
There was one person at my dojo who hit her head on another person's knee who was on the ground and she had to be taken to the hospital. But now she is practicing again.
I think it is the responsibility of nage to take care of uke especially, since, like you said, an uke being thrown backwards cannot see what's behind them.
I personally don't feel that any rolls are necessary, but I think they are better for your body, and they make practice a lot easier.