This is really cool. Rails is currently looking for feature recommendations for 4.1. You (or author) might want to share this with them. I can see it being a pretty popular idea.
This works great for one or two requests triggered by the developer, but fails when thousands of requests bombard a live application split over a multiple servers with multiple rails instances running on each.
This is when creating your own logs with exactly the data you needs becomes valuable, when you can measure what you need how you need.
My first post set the foundation for being able to easily get the right metrics and information by creating solid logs. It does not mean that this has to completely replace the aforementioned tools, on the contrary, it can supplement them to give you exactly the data you need when you need it.