Imagine a user with a smallish Surface Pro (say, 32GB) and/or Windows Phone and another device with a lot of storage (say, 1 TB) . Over time, the user accumulates a lot of files, photos, documents, and music. All this data could be automatically uploaded (if desired) to OneDrive. But, if the user desires to clear away these files from their device, they should remain in OneDrive (online).
Imagine further, this user would like to (from all of their devices) save all of their documents to a single Documents folder (tree) and all of their photos to a single Photos folder (tree) in OneDrive. If the Big computer is syncing, say, 300 GB of images to OneDrive, One-Way sync would prevent the smallish Surface Pro and/or Windows Phone from receiving a flood of files and filling up these smaller devices' storage.
When they need to retrieve a file from online-only storage, they could either use the Web UI or, perhaps, rosebud view/explorer view/WebClient (I'll leave that to your product guys to leverage the right technology).
One-Way sync --> from device to OneDrive (online).
Plus, selective Two-Way sync (which I believe already exists).
Microsoft should definitely introduce on-way backup feature within onedrive.