Ownership of everything stays solely with you, and the rights you grant are tied only to the services and letting them do their jobs.
Dropbox: "You give us the permissions we need to do those things solely to provide the Services."
Google Drive: "The rights you grant in this license are for the limited purpose of operating, promoting, and improving our Services, and to develop new ones."
Skydrive: "you hereby grant Microsoft the right, to use, modify, adapt, reproduce, distribute, and display content posted on the service solely to the extent necessary to provide the service."
The reason you have to give the rights for making copies, etc., is so that the companies can move files around on servers and back things up for you. Rights for public display are if you wish to share with other people, or if you want to make a file public (an image, for example).
Every cloud-storage service uses more or less the same language to offer the same services. There's nothing nefarious about it at all.
http://www.theverge.com/2012/4/25/2973849/google-drive-terms-privacy-data-skydrive-dropbox-icloud
Interestingly, no-one seems to mention Apple's cloud-storage service, and how their terms and conditions give them permission to delete any file they find "objectionable" at any time they wish. Note that "objectionable" is not clearly defined.
If you're really that bothered about security, you can encrypt your files before uploading them, or use a free service that encrypts everything before it's stored, such as
Tresorit.
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I've used Dropbox in the past, with versioning software, for programming projects, and I've just started using it for my WIP. I've also got cloud space with Google Drive, Box.net, and Skydrive.