Yeah, well a lot of SEO people are just parrots who repeat what they've heard.
Does "Amazon" equate to a massive multi-product marketplace somehow? How about "ebay"? "Flickr" is so much like "photo hosting" that you can see why they're such a popular search result. And so on.
Having key search terms in your domain name lends a tiny bit of weight to the overall result ranking. The link text, page title and H1 tags will do more for you though. So will text in the url itself (www.foo.com/these-are-my-keywords).