Why is it a bad idea to trust a cloud (whichever one) as your only storage?
Here's an example
For some reason, an Adobe's Creative Suit update removes all data in the first folder it encountered in the storage space. While you won't lose all data, you might lose an entire project.
The latest update fixed the problem, but if you were under the impression that your data was safe 'because it's in the cloud', then you could get to swallow a very bitter pill... especially if the project you're working on must be delivered tomorrow and is (was...) called "AAA Marketing Techniques for Startups" or something.
edit: From this article, it's not 100% clear to me if the Adobe software removes content on the local drives, or on the cloud storage. It could be that this is a local issue. Still, it clearly shows why having at least one independent backup is important.