"Anyone can change and delete files/folders on your C drive which contain important files/folders in order for Windows to function correctly."
"Anyone can accidentally delete your Windows folder and cause your system to crash or accidentally move files to a different folders cause system to become inaccessible."
I don't think that's a practical problem becasue the default permission level is READ only. Also, Windows 7(and Vista too) prohibits sharing of Windows folder, Program Files folder, and Users folder(other than Users\Public). I assume that people who want to
share the C drives have isolated home environments.