Since MediaWiki 1.18 we have the variable $wgUseCombinedLoginLink [1] which is set to true per default.
During edit workshops with students and seniors I registered that new editors are confused about the combined login page. They tried to register new accounts on the login page.
Surely, these observations are not representative but I think that the usability could be improved by setting $wgUseCombinedLoginLink=false
If I missed a prior discussion about this issue I apologize and would be happy if someone could point me to the discussion.
Otherwise I suggest to set $wgUseCombinedLoginLink to false for all WMF wikis.
Raimond.
[1] https://www.mediawiki.org/wiki/Manual:$wgUseCombinedLoginLink