I want to make sure I understand why you think this pkg_resources adapter help's the migration.

There's 4 things here to distinguish:
1) not changing very much core pip code, for fear we might want to easily revert out of this later
2) changing less code, because it decreases chances for bugs
3) pip having classes/functions on top of distlib so as not to repeat routines over and over.
4) pip continuing to use the pkg_resources interface, because that's perceived as a good thing for some reason? 

what is your rationale for doing it this way in terms of 1-4?

unless we're really motivated by 1, not sure why we'd do this.
we're creating an adapter that we'll want to replace later with a more direct distlib layer.
we don't have that many calls to pkg_resources to replace.