Feature summary:
mw.message(...).parseDom() should be available if mediawiki.jqueryMsg isn't loaded and return a text node with the unparsed message contents by default.
Use case(s):
It's possible to use .parse() without jqueryMsg, but it isn't possible to use .parseDom(). However, .parseDom() is faster than .parse() if the result is used with jQuery since .parse() serializes it to HTML.
In mediawiki.notification for example, the following is used:
$notificationContent.html( message.parse() );
This is suboptimal because .parse() creates a jQuery object, serializes it to HTML and passes it to .html(), which will parse the HTML again. The issue is that it's not possible to use .parseDom() instead since mediawiki.notification does not currently depend on jqueryMsg, and if it did, it could have a negative performance impact. If .parseDom() was always available it could be used here.
Benefits:
.parseDom() could be used more often, which would increase performance and avoid unnecessary (de-)serialization.