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A few more thoughts. The original impetus for this feature was Issue4865. The use case there seem to be from users of Google App Engine back when it was released using Python 2.5. It seems to me that the use of dmg installers for Python packages has diminished; certainly the unofficial packages at pythonmac.org haven't been updated for more recent releases. The issues with installing some packages (like PIL) with 3rd-party C library dependencies notwithstanding, I wonder if part of the original call for this feature was the fact that Apple does provide setuptools easy_install's with system Pythons and there is confusion that a setuptools/Distribute instance, with its own easy_install command, is needed for each Python. Some of that confusion should diminish over time with the availability of "pip -E" and, starting with 3.3, a batteries-included installer command, pysetup.
Also, with regard to backward compatibility, I speculate that there hasn't been much notice of this feature since it only affects users of Python 2.7 on OS X 10.7+. For Python 3, the presence or absence of the feature doesn't affect anyone because Apple has yet to ship a system Python 3 so removing it from 3.3 would have no backward compatibility impacts, unless an administrator manually created a /Library/Python/3.x for some reason and manually installed things there.