Why are all languages selected by default in the MiKTeX 2.9 Settings (Admin)?
In my experience in the previous MiKTeX distributions, only a few languages were selected by default.
Is there any hidden effect of this new setting?
You would, of course, have to ask Christian Schenk for a definitive answer. However, it seems that a plausible line of reasoning is 'ease of use'. Most people never go near the settings boxes, so it's best if the defaults work for the majority. On a modern computer, including all of the hyphenation patterns is not a big issue (which is would have been in the past). So there is no real gain from omitting some languages: as @daleif comments, all that happens is that people are confused.