Last modified: 2014-05-12 19:46:53 UTC
This behavior was changed in 3b147f876a0c7450025d2fe4e8c7117cb2be50d0 . There's always a variant selected in user preferences. Using &uselang= overrides the 'variant' user preference, when it's different from 'language'. This notably affects page preview (tested in live preview), where a &uselang= parameter is added automatically based on current interface language.
3b147f876a0c7450… = http://mediawiki.org/wiki/Special:Code/MediaWiki/60923