I think that is a decision best left to the developer or user. Take an app that syncs your photos from a site you may already use, like Picasa, Flickr or SmugMug. You want to have these available on your iPad or iPhone instantly, even when you are without internet service. But you don't need them backed up to your computer every time you sync, causing the backup times to skyrocket, and if you lose them on a restore, it's a minor inconvenience, but not a huge deal. Losing them unexpectedly when you don't have the time or ability to re-downlaod them is a much more major problem for some people.