I am more comfortable using Diaro as my journal because they don’t keep the data, I do (sort of — really Dropbox does, but it’s my dropbox acct and I can take it/delete it whenever I want). I think that may have actually been my deciding factor in choosing Diaro.
In this particular case, using Dropbox has the added (I’d say necessary) feature of syncing across devices so any apps that store user data there can see it anywhere and not have to worry about managing it.
It’s also interesting to note that this wasn’t really the #1 use case (afaik) for Dropbox. But it does seem to be a natural (albeit relatively fringe) additional use case. And I wonder if we will see an increasing number of apps (maybe health?) take this route, and marketing it to privacy/control conscious users.