Because you’d have to stash your modifications to be able to switch branch.
OP said nothing about stashing, only committing WIP commits to feature branches. I don't think none of your remarks apply, because if you really need stuff from the WIP commits you can also cherry-pick them or checkout specific files.
I agree. I wish they moved that to a standalone section so that it could be easily skipable. Reference docs can and should have a rationale, but a lengthy rant is not what leads people to the site.