They way you are thinking is something that has bothered the community for more than a decade. Yes, you are actually right that keyword based domain names are ranking higher than non-keyword based. Most likely over time this will change. Something that I always seen with Google is that they are trying to implement changes slowly (meaning that over time these keyword based domain would most likely fade away).
But if you look at the issue at a marketing prospective does this client only want to rank for one keyword? Y/N?
- Yes: then go for it! if this eCommerce will not be targeting now or in the future other keywords then stick to a keyword based domain
- No: if you have a lot of different products and you are looking to rank for different keywords, its better to build a brand name for your company by having a brand name oriented domain name!
I tend to focus to the brandability of the domain name (short,catchy, .com, etc) rather than the keyword focus, UNLESS we you are going to target an extremely competitive keyword which by it self could bring tons of traffic!
Some articles worth reading are:
http://www.searchenginejournal.com/how-your-domain-name-will-impact-seo-social-media-marketing/
http://www.motocms.com/blog/marketing/10-tips-choosing-domain-name/