Well, the fact that you are seeing different users in this file lead to the conclusion that the deployment process of configs is broken. Apparently in the correct deployment only cluster specific proxy users should be added to the list.
Another thing about proxy users is that they normally should have a set of hosts from where they are allowed to perform their impersonation. That means that the way proxyusers are implemented for Herriot are error prone because one has to make sure that these users are accurately configured in superusers configuartaion file.
This said, proxyusers file doesn't solve any problems but rather adds new ones. This is good idea though to have an API to extract the list of proxy users from the configuration of a daemon. It also seems to be beneficial to provide an information about allowed hosts for the users in question.
As for gridmix testing: providing a file means that gridmix tests will have to parse it as well. grdmix uses Hadoop Configuration class and the tests should be able to benefit from this fact, e.g. it will do xml parsing and provide all needed API to access its content.
In other words I'm strongly opposing this proxyuser thing. The fact that we have this in the code is partially my fault: I had to think it over better when I was reviewing the patch initially.