As an English Wikipedia bot operator, I am using Parsoid to retrieve parsed HTML versions of Wikipedia articles as part of the operation of my bot. As I'm trying to be a good citizen and not cause the nice people in WMF's analytics department any particular headaches, I thought it might be sensible to set a custom User-Agent string for my requests.
I can only do this by overriding the string set manually in lib/mediawiki.ApiRequest.js - making it so that this can be overridden from the localsettings.js would be quite useful for bot operators who are using Parsoid to retrieve parsed copies of content from MediaWiki installations.
(I may be able to prepare a patch, but filing the issue first in case I don't get around to it.)