Re: significance to testing/belt rank?
In our dojo (at the kyu level, anyway) there is a 'pre-test' where you basically do the test but after/before class with a smaller audience and without filling out the forms and paying the money... you also get comments and errors pointed out. If you pass the pre-test, you will 'nearly' always pass the test test (in class, everyone there, have to pay a fee, same content as pre-test). However, it's comparatively more common not to pass the pre-test or to yourself decide not to test yet after the pre-test.
I suppose it works OK. Practically speaking, I do like the idea of saving the money by having them tell me if I'm not good enough to 'probably' pass. And even if the vast majority pass, the pressure is there on test day in the form of embarrassment (also pressure on pre-test day, certainly!). Everyone wants to have a good test, not just barely pass.