For the WMF Localisation Team, having global preferences is a big deal. However, for this team it is out of scope. Consider for instance this scenario:
The user sets the language of his user interface and when he goes to another Wiki, he will know what the software expects of him because the language of the user interface stays the same.Another nice scenario:
The user indicates what languages he knows in his Babel information and he will by default see interwiki links in the languages he knows.Scenarios like these make sense on any multi-lingual project like Wikipedia or Wiktionary. When we start to make use of what we know of a user, we can be more helpful. The interfaces we build can be less cluttered. More important is that we treat people like people and ask things only once.