Now, I can see the benefits on resources to offload heavy items to other services and while those services are up and operational, its brilliant. Saves you money and means you can serve more users from the same hardware.
However, what happens when they do go down? In most cases, your site should still work just missing functionality. Now, in most cases you will be pre-notified if there is likely to be any prolonged downtime to allow you to do something about it.
The thing that prompted this article was the shutdown of MegaUpload by the US Federal prosecutors. They have had MegaUpload shutdown instantly for violating piracy laws, however, sites like this are used for genuine reasons too – the users have had no time to download any files they may have uploaded there and host them anywhere else. The service was just taken straight offline. This has meant that many sites with hundreds of thousands of legitimate files hosted on this service now point to dead links with potentially no way to get the files back.
What would happy to you if Dropbox suddenly went offline? Or another cloud based file storage solution that is out of your control?