On Wed, Sep 2, 2015 at 8:51 PM, Simon Marlow
Stacks of commits are hard to reviewers to follow, so making them easier might have a detrimental effect on our processes. It might feel better for the author, but discovering what changed between two branches of multiple commits on github is almost impossible. Instead the recommended workflow seems to be to add more commits, which makes the history harder to read later.
I've reviewed+merged various big diffs in the form of branches published as pull requests (on and off GitHub), and being able to see each change separately with its own commit message was way easier than one big diff with a summarized message. If Phabricator would use merge commits, reading multi-commit history, especially what commits got merged together (aka what branch was integrated), is easy. Also, bisecting is more precise without collapsed diffs. Therefore, I wouldn't say the single-commit collapsed view is the right choice for all diffs.