If you use third-party online storage services, you could be opening yourself up to yet another system of surveillance. A storage provider such as Acer could potentially snoop through your files, or share said files with other surveillance-happy parties (such as the NSA).
You’re better off keeping your files on your own storage devices, especially if you use paid storage services. Flash drives and SD cards are getting large (in terms of capacity, not physical size) and cheap. Forget that $10/month (or whatever a service may charge), and spend that money instead on a flash drive. You’ll probably get about the same amount of storage, but your files will be on a medium you can control.
Of course, if you’re planning to share a file with the general public, nothing on this post applies. Online storage services are probably the best in such cases.