Let's say you are a large business selling hundreds of even thousands of products. You decide to name your business Walmart and put every item on your website. You also figure to add and remove products all the time. Seems to me like a lot of work. I guess some of it could be automated to some extent but we're still talking a lot of resources, both human and monetary.
If there was a good reason to do it this way, they would all do it. Since they don't, I guess it's not such a good idea. Just create pages from the main domain. Create your PPC campaigns and do SEO.
If you have just a few products, it's more manageable. If the product is your own, it would be a good idea to register the domains for each brand, regardless if you use PPC or not.