I believe they will have to add it in the near future as more and more people use iCloud for syncing data and sooner when they drop prices on the storage.
The issue is how to present the UI to users. Dropbox is doing it wrong IMO but they've improved it recently. It used to be that you have to wait to sync everything before you can deselect folders but they moved it up to the front now, so you can deselect as soon as it is installed but it didn't pause the download.
In addition, Apple's biggest problem right now is that iCloud is not being a nice citizen for bandwidth management. It will use all your bandwidth to sync and this is bad. Apple need to understand that not everyone have unlimited plans nor fast speed.
I hope so. I've been using OneDrive for my online backups since I have 1TB, but don't really like it. Dropbox offers NO middle ground (don't need 1TB for $10). iCloud has a middle tier, but no selective sync so it's useless to me.