Instead I think the promise is, "Don't worry your pretty little heads about what code is running where. Trust us to handle that!" Which surely seems seductive until people realized that they still have to think about those things.
I heard about one project where consultants decided to use the sparkletastic magic of AWS Lambda so they didn't have to worry about how to scale up their crawler. But they didn't really think it through, so they just had a zillion invocations sitting around waiting for HTTP responses. First-month bill was $12k when you could have spent ~$100 to get the same results with a low-end virtual server running a basic Scrapy setup. So for Amazon I think it's less about filling in a utilization gap and more about attracting people who don't know how to optimize.