When I first started looking at virtualmin I could not understand what usermins role was especially when in context of virtualmin as a hosting platform as usermin does a lot more than webmial.
Virtualmin is split across 2 distinct areas, App hosting and web hosting. For web hosting usermin should be reduced down to a webmail client and probably left with more functionality for Apps. Obviously usermin can still be configurable by the user. I did a feature request where usermin options can be configured in the server templates (I am sure I did but cant find it).
There is some replicated functionality such as file manager which is present in usermin/webmin/virtualmin
I am not a linux guy so my views are skewed because I am just using virtualmin as a webhosting platform so maybe I am just not seeing the full picture. 
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