First, in the spirit of full disclosure, I have to admit that I've only tried a spinner once or twice, and didn't care for the results.
Now, with Google finally taking action on machine spun content (at least that's the rumor, we'll still have to wait to see how it all plays out), it got me to thinking about the essential difference between spinners and humans.
A spinner can only work with the input it's given. This input can come from the person using the software, as well as preloaded information (such as lists of suggested synonyms). It will then churn away creating as many variations as you want it to.
However, where they really fall short--and why Google and people can detect them--is in their ability to change syntax and tense.
Humans shine in this area, and I also believe a good rewriter is virtually undetectable (note the use of 'virtually'). I even put this to the test a while back, and Warriors did no better than chance when guessing which articles were original, and which were human rewrites.
For this reason, I think spinners will continue to fall short of human ability for quite some time to come.
So, is that the main difference? Are there others? Or am I full of beans?
(Like I said, I am not initimately familiar with spinners, so I admit I could be way off).
All the best,