I had the water in the ear thing done too, but thankfully, not the rotary chair! Yes, I like roller coasters, but for some reason, that sounds like some sort of torture device!
I think the tests can give them a better idea of where the AN is - is it more on the balance side or more on the hearing side. It also gives them an idea of how damaged that portion of the nerve is which can help them give you a better idea of what things will be like afterwards. In my case, the hearing portion was not affected, but my balance nerve was only functioning at about 40%. That let them know that since I was not having any real balance problems pre-op, the other side had already been compensating quite well for the loss. My doctor was able to warn me that my adjustment to the loss of hearing would likely be worse than my adjustment to the loss of the already damaged balance nerve. He was right. My balance now is pretty good, all things considered (as long as I don't turn my head or make any sudden moves!).
Maybe their thinking is that since the entire nerve is not being removed in one having radiation, the extra tests may not be necessary.
Lori