* Dropbox doesn't guarantee 11 9s like this service does - when I'm backing critical data up, I want to make sure it's _there_. * Dropbox likely wouldn't take kindly to me storing 10PB, whereas that is what this service is designed for. * I've got SLAs and guaranteed speeds with this, Dropbox isn't designed for me to suddenly download 10PB very quickly.
Those claims are from glacier: http://aws.amazon.com/glacier/faqs/ which is priced the same per GB of data.
In this case, the durability refers to the loss of data/objects stored per year - if you're sending multiple PBs of data off to Glacier, you want to be able to retrieve them many years later. Even 5 9s would mean that 1 object out of 100,000 is lost every year, which is quite poor.
http://en.wikipedia.org/wiki/High_availability#Percentage_ca...
In this case, it's not as much service uptime as it is data retention. If you're storing 5+ PB of data, even 5 9s of data loss per year can have a measurable impact.
source: SE at Google Cloud
Also retrieval speeds increase with your data set size: Note: You should expect 4 MB/s of throughput per TB of data stored as Nearline Storage. This throughput scales linearly with increased storage consumption. For example, storing 3 TB of data would guarantee 12 MB/s of throughput, while storing 100 TB of data would provide users with 400 MB/s of throughput.