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Too bad you didn't find this on one of the RCs. The fix will have to wait for 2.7.1 now.
The line you originally quote as changing was, as far as I can tell, the original code (it enters our repository on 2006年04月22日 in r45654 when optparse was upgraded to version 1.5.1 of optik). The change to the plural form in that line was made for 2.6rc1 in r71345, possibly inadvertently since that changeset was the bump to rc1.
It would seem reasonable to change this to match 2.6 for 2.7.1, since as far as I know no bugs have been reported against it in 2.6, and it is unlikely to break working code in 2.7.
As for the more extensive problems, now that argparse is part of the standard library it would seem better to devote any development resources to it. Can you test your issues against argparse, and open a new issue if it also has problems?