It's not about saving though, it's about reading files that already exist in certain locations.

Imagine for instance an application that helps you clean up your downloads folder, but wouldn't be appropriate to use on other folders on your system. It's not the best example, but it illustrates how much of a usability mess it is to ask the user to locate the folder for you through an open file dialog, and then have to reject it if they select the wrong thing.

That's a fairly niche case, though. Personally I'm comfortable sacrificing it in favor of not giving apps full read-write access to everything on my system by default.

I can see how there are very specific cases in which it would be too cumbersome, but it probably works in > 80% of cases, and it's pretty clear that those 80% apps are who the App Store is aimed at.

Edit: Thinking about this some more: not to pick on your example, but the Downloads folder actually does need to be located by the user, because it's not actually in a fixed location (it defaults to $HOME/Downloads, but that's not where mine is located, for instance).

There really aren't many cases that I can dream up in which you can absolutely be certain of a file or folder's location, such that an Open Dialog is totally superfluous.

It's not really niche though, the entire Utilities and Dev Tools section of the app store are full of these kinds of apps that provide users with convenient management of data in predetermined locations, my own included http://itunes.apple.com/us/app/space-gremlin/id414515628?mt=...