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CPU usage is very low, bandwidth usage is low due to caching, and storage depends on the size of the community (example: 4 MB maximum per 1.000 users).
The extent of copies depends on the overlap of communities (I do not think that this is very high). However, you could link the cache directory to a single place.
Finally, the proxy and caching functionality can be disabled.
So, there aren't any grave technical arguments, but I think your rejection is rather political. But, of course, that's okay.
Dear Developers: Thank you for the great work!
However, I think I've found another regression bug:
Call to undefined method ProxyRequest::status() in library/core/functions.general.php on line 759
This call is inside the function
FetchPageInfo, which is used for Vanilla Comments. Consequently, in 2.1.13p1 it is not possible to write the first comment to an article using Vanilla Comments (the user gets the error message "There was an error performing your request. Please try again.").
It is possible, that the bug was already introduced in an earlier version. However, 2.1.10 worked well.