After reading the OP, the tradeoffs seems more complex than a single origin in us-east and having CDNs cache HTML pages.
The most appealing advantage I see with running rails at all these regions is you can authenticate users really quickly and redirect them to a html page in a CDN.
My general takeaway was that HTTP is a terrible interface for "programming" things. Worse, the whole stack was difficult to test and inflexible to iterate on. And the apps I've found myself wanting to build lately rarely serve the same HTML twice.
You may find differently, but I strongly believe "just run your app and DB, no other layers" is the simplest possible way to do most full stack work.
i have to try it now. After reading the free postgres and last mile redis blog post, i realized something...
I can use probably the most under-used cache stores rails provides: the file_store, which is twice as fast as trying to read from a memcache/redis process. This was not even an option on heroku b/c of the ephemeral file system.