5000 reads per sec of 64Kb items, would make you stream 2.5 Gbits/sec using consistent reads and 1Gbits/sec writes, moving close to 1.5TB each hour. At the end of the month you have read well over 800 TB and updates 160 TB... That is a substantial application you have in mind... :-)
That may be true for an application with a constant load, but applications with a less balanced load have to provision for their peaks. My company (Malwarebytes) has very irregular traffic (at the hour mark we get very big spikes, but only for a couple of minutes) and it seems like we would have to provision (for this specific app) that peak for the entire hour. I might be misunderstanding the billing for this service though- if we ask for more units for 15 minutes, would the billing be prorated?
This actually hits on my only real issue with AWS in general, which is the hourly billing. We've used the mapreduce service a bit, and having a cluster roll up and fail on the first job is heartbreaking when it has a hundred machine hours associated with it. Obviously that is far, far cheaper than us building out our own cluster (especially with spot instances, which I can't even describe how much I love), but for some of the services smaller billing times would be useful.