Call me daft here, but there has been a lot of talk about the URL rewriting that WordPress does behind the scenes for those of us that use friendly-URL's. As I understand it the rewriting is done using SQL queries and a fair old whack of PHP code to process and decipher the URL and translate it into a category or page or post or attachment etc.
However, using rewrite rules inside a .htaccess file is a lot lot lot faster. Mostly because it's done using the Apache (or IIS for Windows servers) low-level engines rather than the higher level PHP, MySQL and ultimately WordPress applications and software. I know from experience that .htaccess rewrites are lightening fast and can handle huge amounts of conditional statements and URL options in a very short space of time, without adding any overhead to page load times or server resources.
If WordPress were to generate a .htaccess rewrite rule set for the specified friendly-URL then there would be no need for expensive SQL queries to determine what the URL specified should relate to. The rewrites would reformat the incoming URL's into a single, predetermined order that the WordPress software does not need to do any more processing with, thus streamlining the whole process of retrieving a specified page/post/etc.