You will definitely need to add content to an aStore page; otherwise, there isn't anything for a search engine to index. They do not index content in an iFrame. Even if they did, it would be the same content as 10,000 other Amazon sites.
I just set up my first site with an aStore a couple of weeks ago and I added content below the aStore. We will see how well it is doing in a couple of months.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin