If I'm searching for programming stuff or similar, I want it off. If I'm searching for shops that sells bricks for my new wall, I want local results.
I've also noticed that even with it on, the "local" indexing is distinctly significantly worse than Google. That is, Google is much, much better at finding relevant pages which may contain variations on my search term when using my native language.
I guess that's just a question of resources, which is fair enough.