While resarching what happens with OneDrive sync when files are deleted in the Desktop OneDrive Folder I discovered something about plans for Smart Files for Windows 10, which may well affect decisions about (if and when) to upgrade to 10. To clarify, Smart Files mirror files online but do not take up space on the HDD. They are represented by placeholders and can be 'viewd' even though the file is not actually there! This saves space on the Hard Disk, and for me (and presumably others) with my small Drive on my Tablet is very important.
It appears that Microsoft is planning not to implement Smart Files when Windows 10 is launched for the public. This has alarmed some people, some of whom are threatening to abandon OneDrive altogether because of it ---- 'when Smart Files go, I go' is a common response. However, Microsoft has put some effort into explaining two things: a replacement for Smart Files will be introduced later (this year) in an update, and in the meantime there will be an alternative way of avoiding files on OneDrive taking up space on the local Disk. As I understand it, details of this are not yet available.
I am not at all technically savvy (I have only just discovered Smart Files and making files only available online while still visable locally; and the benefits of this.) So if I have said anything which is either incorrect or misleading about this I welcome being corrected, and I would welcome any further information.