There is also the option to spin capitalized words (assumed to be proper nouns) as well as leave any number of words unchanged, depending on whatever you enter into the "ignore" field, separated by commas. You also have the option to only keep the sentences that were altered a minimum percentage, as indicated by the "Keep Sentences that Changed" option.
Think of it this way: What do you have more chance of hitting, a distant, tiny target or the broad side of a barn? For the sake of search engine optimization, you want your website to be the broad side of barn. The more quality articles, or unique readable content, your website or blog offers to search engines (and therefore the general public) the more exposure your website will receive from major search engines.
It looks like the creators of Spin Rewriter 9 .0 have really put a lot of work into creating a piece of software that creates readable content. However, Google isn’t stupid and there are a lot of things that can go wrong when you are trying to “manipulate” Google into giving you more traffic. Google hates spam, badly written content and duplicate content. These are all things that Spin Rewriter 9 .0 is capable of creating. You simply can’t trust a piece of software to write content for you in my opinion.