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.
