I understand that, but I don’t think that it’s that clear to our ‘average’ user, who might start with a subset on the web, or a tree with placeholders, for privacy, and has no idea that this can happen.
I have two commercial desktop programs that can sync trees, Family Tree Builder, by MyHeritage, and RootsMagic, which can sync with Ancestry, and they both only allow the synchronization of full trees. And I bet that they have good reasons for that.
Ancestral Quest, Legaccy, and RootsMagic, and some Mac programs too, can download branches from the shared tree on FamilySearch, but that is a one-way street. Users can not upload such things, to prevent messing up the shared tree.