I'm not very familiar with this issue, but a quick review of the patch and the existing comments lead me to believe that commiting it as is wouldn't be the cleanest way to deal with this problem:
1) As Mark mentioned, a "disableCoord" option has been added to BooleanQuery which makes the ExpandedTermsQuery class in this patch unneccessary. furthermore, the ExpandedTermsQuery class is (in my opinion) broken because it ignores any user specific Similarity completely (instead of just ignoring the coord factor)
2) This patch does not include any test cases.
Furthermore, this patch (assuming it does what it says it does) would fundementally alter the scoring of a FuzzyQuery – the new scoring may seem like the right thing to do for you and for Mark, but without a larger concensus I'm a little worried that perhaps there are just as many FuzzyQuery users out there who are happy with the current behavior and would consider this change a bug. So any "fix" for this should either be configurable, or have an overwelming amount of support that the change is the "right" thing to do.
(The main issue seems to be that terms with low idf have a larger impact, and that results in rare mis-spellings getting higher scores – but assuming "clean data" with no mis-spellings, scoring "rare" terms higher seems like the ideal behavior, correct?)