Here's a comment I wrote about the situation on reddit:
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Yes, I'd prefer SailfishOS to be 100% FOSS, but Jolla is helping GNU/Linux establish its roots on the mobile.
How is it doing this?
* libhybris has opened up a whole new world for GNU/Linux, by allowing programs developed against glibc (i.e. GNU/Linux) to run on top of proprietary Android drivers (i.e. phones).
* Jolla puts a lot of development into lipstick, which makes it easier to develop UXs for phones/tablets/other form factors [This involved developing Wayland support]
* Jolla have opensourced their browser (Qt5 + Gecko engine) and their office suite (Nokia's Calligra Office work, with a QML interface)
* Jolla develops a sychronisation framework (buteo-sync) that can be used for syncing emails, Twitter, CalDav/CardDav, etc (I think Ubuntu Touch might adopt it in the future)
* Want to see how much manpower Jolla dedicates to developing FOSS? Look at the number of @jollamobile.com emails here: https://git.merproject.org/groups/mer-core/activity
* The SailfishOS community has been developing sfdroid. A FOSS application for running Android apps on your GNU/Linux mobile.
So while SailfishOS isn't 100% open source, it's doing a lot of good for the GNU/Linux community on the mobile.