A key thing with this service is that it's FREE for mp3s you buy from Amazon. From their help pages -
"All new Amazon MP3 purchases saved to Cloud Drive do not count against your storage quota."
So, if I buy mp3 files from Amazon, I have a free backup AND and I can play them across any Internet connection or from an Android phone. This puts Amazon's online music store on a more competitive footing against Apple. To date, Apple's advantage has been its integration of Store + Music Management + Portable Playback. For the growing number of Android owners, Amazon's store offers a comparable level of integration. I may even draw some iPod/iTunes users to Amazon.