The reason thousands of people have used it successfully is because most people are not developers, aren't lazy and willingly accept having to do something inefficiently over and over again. If there is some flaw in the way the system works then people will adapt and go around the mountain. They won't even think about this and, if it's so automatic that it's buried in their unconscious, they may never even think to tell you.
The other reason people have used it successfully is because they assumed that the broken output was correct. If it's not tested, how do you know that your report generator is spitting out the correct figures?
Also, kind of related, just as one of the benefits of unit testing is that you write well structured and testable code, one of the benefits of functional testing is that you think about the interface. I'd consider any inefficient interface to be a bug.