Review requests are useless until GitHub makes the reviews themselves useful.
Right now the greenest guy on the team can approve a PR and it can be merged.
The real dream would be to be able to set the requirements on a per-directory basis, but I doubt that would ever happen.
(Disclosure: I'm the founder.)
Now that we have this tool I wonder if there is an API to add a reviewer. It would be fairly trivial to add this if there was an API.