The way your browser’s URLs return are also important in the Search game. Google arbitrarily refuses to index URLs containing “?id=”. Many argue about how URLs should be written, so that’s why mod_rewrite let you do it whichever way you prefer. You can create URLs manually or dynamically in several formats:
- http://mysite.com/p1234.html (or .cgi or .jsp or .aspx or .php or .cfm or .anythingyouwant)
- http://mysite.com/1234 (no extension)
- http://mysite.com/page-name.html (manually created)
- http://mysite.com/page_name.html (automatically created from the page title)
- http://mysite.com/some/subfolder/pagename.html (uses site hierarchy and allows for duplicate aliases in different pages)
- any combination of the above
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Wordpress and Modx are two amazing free software (Blog and CMS) which handle pretty good rewrite rules for Dynamic URLs.

Thanks i found that really useful, do you have any idea why google does this please?