Well I'm often working much more in bug fixing than feature creation, so being able to have a easily readable git diff is paramount to that. Going on Github, clicking through things etc makes it definitely not efficient ^^'.
And yeah bisecting, is helped by that, beyond the fact that you have a smaller chunk of code to point to when it finds that something is wrong.
That been said, I would partially agree with multiple separate PRs, although as you've said you lose a bit of coherency and requires more coordination, while a single PR still lets you to select the single commits to see the partial diff of each, if you want to not have to look at everything together.
In that case it would be the job of the PR owner not to create commits that introduce something and then modify it a commit later, which clearly then will make it difficult to understand what's the end result.
So as I understand it, you're not against multiple commits for feature X, you just want a different view Github wise mainly