Google attempts to spider dynamic URLs, but has difficulty doing so in some cases. We have found that if you are able to rewrite URLs properly, its definitely adds value.
Google does a good job typically when following a one or two parameters URL (such as index.php?p=home), but, when you have a long query string that has more than three parameters it is often worth the effort to rewrite it.
Also, now with the new link tag (http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html), you can tell Google what page should be used to avoid duplicate content issues.
Rewriting URLs can be complicated and mistakes are sometimes made, so this new tag is really great for SEO. Just put the new tag in the header of the page and you no longer have to worry about duplicate content issues when rewriting URLs.