Search Engines (the important ones) now prefer that the dynamic URL is retained. There has been a lot of discussion about this over the last 4 years, but if the site software is well-built, then SE's will have no trouble following a dynamic URL and indexing it properly. Zencart is such software.
Some "SEO modules" may actually do more harm than good. If they are not extremely well constructed, and function perfectly, you can risk creating "duplicate" URL's, and therefore "spoof" a SE into thinking that you have duplicate content - for which there are penalties.
I have built over 100 zencart sites and NEVER ever used any of these so-called "SEO modules". And (through proper, detailed and focused SEO) some of my clients enjoy significant prominence on SERPS for critically important keywords.
As Glenn points out, no mods are "essential" - but I would argue that there is a lot to be gained from SitemapsXML - as this dovetails with the major SE's indexing preferences and allows fro efficient management of your relationship with the SE's...
This has to be infinitely more advantageous than any SEO module...




