I suspect this is causing some disturbed sleep patterns at Dropbox, but you're right - without the "mounts as a folder and auto-syncs" behaviour that Dropbox has done so well, this isn't going to cut into their profit margins yet.
Right now, this is not super useful - those of us who always early-adopt stuff like this probably already have Zumodrive and Dropbox accounts, as well as our own S3 accounts. In it's current form, this is not quite as useful as Zumodrive, which I've stopped using because Dropbox works so much more nicely.
If I were the Dropbox people, I'd be busy keeping the development moving forward to make it even easier for non technical people to understand what they are and use it, to ensure Amazon are still behind when they finally catch up.
(I'd also consider seeing whether I could match Amazons "mp3's purchased thru Amazon are stored free" offer - I'll bet Amazon are doing that with some smart de-duping filesystem magic which means they only have to store one copy of the mp3 file. I wonder if Dropbox are big enough to be able to gain major wins from that too...)