You bring up a good point, which is what I keep wondering too - where do they go? Dropbox already has sync APIs [0] across both iOS and Android (and presumably desktop), which must be their big bet to get to the central cloud store that you mention.
The part that is lacking is the metadata layer for pics/audio etc which allows apps to read others files, but that could be an easy layer on top of this API.
The part that is lacking is the metadata layer for pics/audio etc which allows apps to read others files, but that could be an easy layer on top of this API.
[0] https://www.dropbox.com/developers/sync