As others have said, being open source is important because it gives you more control over your data. If you have access to the code that is storing your data, you have the option to host it yourself or pay someone else to host it with the same client. I've written about this more on my blog (http://programmerthoughts.com/openstack/democratization-of-d...).
If you are looking for an alternative to S3, I'd ask that you look at Openstack swift (http://swift.openstack.org). It's 100% open source, proven at scale in production, and, if you are hosting it yourself, can offer lower op-ex than using S3 (of course, there is a cap-ex cost to buy your hardware).
If you have access to the code that is storing your data, you have the option to host it yourself or pay someone else to host it with the same client.
That's certainly true in 'regular' open source economics.
But storage is somewhat unique in that the significant cost factor is not the price of alternative proprietary software but the hardware costs of the storage medium itself - and those are constant regardless of the license structure the software layer is using.
Additionally, scales of economy come into play to such an extent that the costs of hosting this kind of storage myself will be wildly more expensive than a volume player like Amazon who operates entire datacenters (this argument goes beyond just the op-ex/cap-ex tradeoff)
If you are looking for an alternative to S3, I'd ask that you look at Openstack swift (http://swift.openstack.org). It's 100% open source, proven at scale in production, and, if you are hosting it yourself, can offer lower op-ex than using S3 (of course, there is a cap-ex cost to buy your hardware).