The core question that you're getting at is really "does it ever make
sense to partner", and to hear you say "no" because "we just need to
do a bit of engineering work" makes me think you are not appreciating
the opportunity costs of doing that work - everything we choose to do
means that there are other things we can't do, or can only do poorly.
(That Pocket is "closed source" is a red herring - most services that
most people use online are also "closed source", and while it might be
nice for that to change, there are plenty of reasons why it won't
easily, and I don't think it should be at the top of our list of
concerns, particularly in the context of enabling experiences that
people love. We cannot refuse to integrate with closed-source
services, because we'd be refusing to integrate with the majority of
the web, and down that path lies irrelevancy.)
Gavin