Wouldn't that be kind of unnecessary, when it's not too tough to write an SQS-reactive client?

Starting with S3 fills a gap as there is AFAIK otherwise no way to start processing based on changes to S3, without writing a rather wasteful program to list the resources, maintain state, and hunt for differences.

But if you have an SQS reactive client it would sit in EC2. With a service like Lambda, they are promising millisecond based pricing. A very ambitious project by them.