I am not really concerned Microsoft is downloading files from computers, but I am concerned if they technically have the ability to do so. If it requires an encryption key or some kind of password, these have a tendency to leak. If it is limited to a certain group of people, who's to say none of these people can be persuaded to sell your data? And would it be acceptable if the three-letter agencies can start snooping not only in the cloud but also on your own computer without you knowing?
I remain unconcerned.