I was going to change the default, and i'd even already written up the CHANGES.txt verbage to include in, when i noticed that it caused 2 tests to fail: on for DisMax and one in ConvertedLegacyTest.
This wasn't a huge surprise, i figured the test were just expecting "broken" behavior, but when i looked at exact failures they were by no means obvious failures. In both cases doing "the right thing" had some subtle impacts on the matching/scoring of docs that made me realize changing the default is probably not in the best interests of existing users (if it caused problems like this in our simple unit tests, it could have some pretty serious impacts on real world cases)
FWIW, here's the verbage i was going to add...
A new "enablePositionIncrements" option has been added to the
StopFilterFactory. The default value is "true", indicating that a
"gap" should be left when a stop word is removed, which will affect
how much slop is required in order for Phrase Queries to match. Users
who wish to preserve previous behavior should add
'enablePositionIncrements="false"' to usages of StopFilterFactory in
their schema.xml. Other users should consider reindexing to ensure
consistency in behavior for all documents.