Adding to the previous post there are two additional potential problems when using dynamic URLs.
Search engines have problems indexing URLs that contain session IDs. I would only pass session IDs in the URL in areas of the site which are not to be indexed -- such as password protected areas or shopping cart pages.
Session IDs cause problems with the SE bots. The session variables are different each time the bot lands on a the "page," giving the impression that the page has a new URL every time it is visited. This appearnace of duplicate content causes numerous problems, simply put Session IDs must not be visible to search engine.
A second problem with Dynamic URLs come in parameter ordering. The coders must be careful to order the parameter the same way each time else the search engines will have to juggle which "url" to use to go to the same content.
All in all dynamic urls are fine as long as no session variables are used in indexed pages and if the coders are consistent with their parameter ordering.