The use case that made it necessary for me was the gmail client's long refusal to let me "save" attachments. I knew it had to be downloading them somewhere to open them in whatever preview app used (Adobe, etc), it just refused to show you where it put them, and make sure you could save them for later (esp useful if you have spotty data reception).
That said, I don't reject Aaron's ideas out of hand - I'm really looking forward to the KDE tablet, put the problem is usually apps that decide "the user doesn't need this, they have the cloud" or something dumb, and the only way to get to stuff is via the good ol' filesystem.
Assuming KDE let's you find everything, and let's you open files in programs you're not "supposed to" (e.g. binary files in gVim or some other "why would you do that" type of thing that I occasionally do have to do!), then I'm good..