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In their Webmaster Guidelines, Google actually specifies that you should make your URLs just a string of relevant words
the biggest benefit I found is exactly what deadsea said - minimising endless duplicate pages created because of inconsistant use of parameters
You should make sure the words are checked and such incorrect URL requests either return 404 or redirect to the correct URL
which means URL rewriting has no siginficant effect on SEO by anyway and only helps to control URLs duplication?
A site's URL structure should be as simple as possible. Consider organizing your content so that URLs are constructed logically and in a manner that is most intelligible to humans (when possible, readable words rather than long ID numbers). For example, if you're searching for information about aviation, a URL like [en.wikipedia.org...] will help you decide whether to click that link. A URL like http://www.example.com/index.php?id_sezione=360&sid=3a5ebc944f41daa6f849f730f1, is much less appealing to users.
Consider using punctuation in your URLs. The URL http://www.example.com/green-dress.html is much more useful to us than http://www.example.com/greendress.html. We recommend that you use hyphens (-) instead of underscores (_) in your URLs.
Use informative URLs. The URL (web address) of a page appears below the title, with words from the userís query in bold. Your URLs should be simple and human readable. Which do you find more informative: http://example.com/products/shoes/high_heels/pumps.html or http://example.com/product_id=123458?