It's way easier and less error prone to specify a revset after importing a
series than to manually type in a series of Differentials.
Unlike most revision oriented commands, this requires the --rev option
explicitly because the existing DREVSPEC doesn't need to have the leading 'D',
and therefore the meaning is ambiguous. I wouldn't have a problem giving
precedence to the local revnum, but phabread and phabimport also use
DREVSPEC, and local revisions make no sense there. I would be fine with
modifying that definition to require the leading 'D', but I'm not sure how many
people are used to not specifying it.