Use Case: In may scenarios I don't want to sync all of my notes. My work computer only needs my work notes. My phone only needs my todo list. In addition some of my notes are more sensitive than others like my journal which I would not want to be sync'd to my work computer. In this case there should be an option when setting up Joplin cloud to choose which notebooks are sync'd to your local Joplin instance. This would also make Joplin cloud more attractive as a service as other cloud sync options do not have this feature.
Thanks for your consideration.