If you have a loader on your tractor...I would say get the spinner, you will be much more efficient and like what Ray said, will free one hand up to work the loader all at the same time.
I have one on my old manual steering tractor and it works great. It seems like the steering is 40 turns lock to lock and when using rear implements I can't imagine being without it. I'd be constantly turning back around to turn the wheel with both hands, and then I wouldn't be able to watch the implement...
I have a spinner on my L2800 and love it, I was getting raw palms before the spinner. I don't have one on my non-powersteering mitsubishi but I don't have to turn the wheel like I do on the L2800.
Watched a buddy break his wrist on the spinner of his CJ5 (no power steering) one day. Short time later dad caught the front tire of the Massey (30hp) in a rut and did the same thing. I wouldn't have one on rough ground without PS.....JD sells a nice one for those of us with green tractors..Just never bothered to get one as it would probably catch in my jacket.
I can see having one on non-power steering like my neighbor's old JD. You turn and turn the wheel without much happening.
However, on my tractor my main problem is often turning the wheel too much too fast. The power steering lets you turn to full lock in the blink of an eye. I don't need anything that speeds up the turning of the wheel on my 4010.