Apart from the free catalogs, which are comparable between the two services, Amazon also has an enormous paid streaming catalog. Like most nerds, the value of an hour of my time is such that even thinking about the price of paid streaming content is a waste of time, so Amazon's paid streaming catalog is a great win for me; 9 times out of 10, whatever I want to watch, I can get through Amazon.
Amazon also does a better job of maintaining my library of past purchases and my wishlist than either of Netflix or iTunes.
I didn't buy Prime to get the videos, but the videos aren't s small feature. Amazon executes streaming content as well as anyone else, and if you haven't checked them out (because you use Netflix, for instance) you should.
Also: I don't think streaming media is at all out of place in Amazon's core offering. They started with books. Then digital media, just like Borders did. Then "everything else", which is what people seem to think of Amazon as now --- the Walmart of the Web. But: then Kindle (consolidating their reach into content), then streaming media, then publishing.
It doesn't seem at all weird to me that the Internet's largest retailer of paid content would have a streaming media service; online content delivery would otherwise obsolete one of their original offerings.