My company has been working with eCommerce storefronts for close to 7 years and this would be a deal killer for me. Out of all the shopping carts on the Market, Magneto is HANDS DOWN the greatest and offers the most scalability for clients bar none! It is more complicated to setup than most but once you have the hang of it you can get a store built out really quick and in the end its just so much better.
I personally, have worked with infopia, zencart, yahoo stores, volution, channel advisor (crooks), 3Dcarts, pinnicle cart and more.
Bottom line, URL Structure in eCommerce is as crucial as Title and Meta Descriptions.