The disadvantage of using the user language is that it reduces
cacheability: all responses have to be marked anon-public-user-private
instead of public where public would otherwise be allowed. True, there are
a number of other reasons that an API response may not be cached, but
adding one more to the pile doesn't improve the situation.
When a client really does need output in the user language, it seems simple
enough for that client to specify "&uselang=user". I note various
JavaScript in mediawiki/core has already been specifying uselang with the
value from mw.config.get( 'wgUserLanguage' ) even before uselang was
officially recognized by the API.
I note that, while adding uselang as an official parameter was done along
with the help change, it was only done then because uselang was on my list
anyway and without it I would have had to add uselang to both action=help
and action=paraminfo (only to remove it later).