It's an argument that could easily turn into a brawl at the local Web consultant pub. Should you keep dynamic URLs (like http://www.merchanttutorials.com/ mm5/merchant.mvc?Screen=PROD&Store_Code=MT&Product_Code= WR101&Category_Code=WeddingRings) or redirect to shorter, "static-looking" URLs (like the above changed to http://www.merchanttutorials.com/p-WR101.html)?
The argument in the past has been that Google did not spider dynamic URLs, assuming they were constantly changing and therefore not worth spidering. Then, when Google started spidering dynamic URLs, the argument became that static URLs do better in rankings. Both were true for a time, but it's clear that Google spiders do fine with both static and dynamic URLs these days. The questions now are whether putting key words into URLs (like the product name instead of the product code) helps rankings, and whether the key words being closer to the root (less slashes) helps prioritize them for rankings. At this point, nobody really knows as Google has not told us.
In September, Website Magazine published an article stating that Google has, in fact, recently said they prefer that dynamic pages be kept with dynamic URLs instead of being rewritten to static-looking URLs. While it's nice to care what Google prefers, they did not say that dynamic URLs would be better than static for rankings. The Website Magazine author's conclusion was that usability still makes shorter URLs better, as customers prefer them. A commentator also mentioned the value of the shorter, more readable URLs, for easier-to-understand analytics.
Another argument that has ended was that having static-looking URLs on a site that also delivers the proper content via the Miva Merchant standard dynamic URLs will cause duplicate content issues with Google and therefore punishment in rankings. Google has clarified that this is not an issue at all so no more worries there.
One reason to keep dynamic URLs is, frankly, that it's easier to maintain that way as they are simply what Miva Merchant delivers. If you're confused or having trouble creating static-looking short URLs, then you can now be comfortable knowing your content will still be spidered by Google. You don't have to worry about short URLs.
Now you want to know what MY conclusion is, right? As is usual, I am not going to give a definitive answer. There are pluses and minuses to both methods, and I don't want to get punched the next time I'm in the pub. But, for my sites I still rewrite the URLs to static-looking URLs with key words in them just in case it helps, and for usability. Nothing indicates that it hurts.
If you decide to rewrite URLs, either to change your dynamic URLs to static-looking, or to go back to dynamic URLs, be sure that your rewrite code is done properly so you don't lose any rankings you already have. If you don't know what I mean, ask an expert. This article also gets into rewriting but it's not for amateurs.
Miva Merchant 5.5 has excellent SEO Short Links capability built in. I have tutorials in the Full Tutorial subscription that show how to set them up.
By the way, I definitely suggest you subscribe to Website Magazine and check their blog often. It's a fantastic resource, and it's free.