I don't see it as "single point of failure", at least not when your using Dropbox syncing - I've got versions of Dropbox running on two phones, an iPad, two laptops, my iMac, and my home theatre box. I've also got a copy of the data available from Dropbox's servers if I even need. (And there is a risk of a corruption to the synced-everywhere datafile propagating to all those places before it's noticed, but 4 of those copies are backed up with Time Machine (in two different physical locations), and Dropbox stores archived versions for me as well).