Implementing interoperability with all the possible cloud storage systems shouldn't be left to each app developer separately. This should be a feature of the operating system.
As an Android user, I'm genuinely interested if iOS users find the sharing options between apps too limited. Do you often end up requesting new sharing options from the developers of your favorite apps?
Also, not every piece of data is a file I'd want to save to Dropbox. For example, I share article URLs from Flipboard to 2cloud many times a day (2cloud opens the URL on my desktop browser). I'd hate to have an extra save/open step between the apps.