Tons of mobile apps do this. 1password is probably the best example, but just the other day I decided I wanted to be able to view a Sketchup file on my phone and the app I found ( https://itunes.apple.com/us/app/sightspace-free-d/id56721967... ) had the option to load files from Dropbox.
My comic book reader uses Dropbox to sync, my todo program uses Dropbox to read my todo.txt, I use Dropsync to do 2-way syncing between a directory on my phone and my computers mainly so I can drop reciepts + other PDFs into a directory on my computer and get them from my phone, tablet or other PCs.
I could go and replace all of these with another tool, but I would have to have a compelling reason to do so and would probably have to find some new apps (or the apps I use would have to integrate a new service)