Interesting, it seems like madness to me to have it the other way around. Why bother with a merge commit if all the changes are squashed into one anyway? Why not just fast forward merge as if it was a commit directly to the main development branch?

Granularity helps with bisecting issues.

at least in github after you find the squash commit equivalent to the PR you can restore the branch and bisect the rest in there. This is much faster!