The benefit in this case is high availability with low overhead. In this config each mailstore can answer for the web requests and do their redirects. This lowers the overall stress on a single mailstore acting as the redirector. It does work because the mail store will redirect the request to the correct server. In our case 1 out of 4 requests will hit the users actuall mailstore.
As far as IMAP proxy this has great bebefit as it removes the proxy as another single point of failure.
Yes you still have the mailstores as single points of failure, but by adding additional mailstores you spread the risk across your cluster. In our case we would only affect 1/4 of our population if we had a failure on a mailstore. For me this is a lot better that a complete or 50% failure.
Also given that this on a Xen infrastructure we can afford to deploy this like it is as adding additional VM's is a LOT less work than adding physical HW. Plus we remove HW as a point of contention if there is maintainance or physical issues with the HW.
This is our approach to this and our millage may vary, we have been called crazy for doing this and perhaps we are, but so far so good.