It would be nice to see someone reply to this question with actual data (pros and cons) than just a simple answer of dubious motivation by a Magento MVP.
Magento seems to be pretty good for SEO, with the web server rewrites changing lots of GET parameters into SEO-friendly file paths. Unfortunately, the rewrites are not used everywhere, but I would consider that a minor issue compared to the struggle of actually getting products imported into the Magento system.
I just went to the oscommerce website and looked at a few of their example sites.
Here’s one that has SEO-friendly urls in the address bar:
Here’s one that’s not so good: