Side note: What is it about graduate work that burns people out on their field? I've met quite a few people with masters and PhDs who switch fields afterward and basically run away.
As "defense" of PhDs program I could say that it definitely trains your mind to look at things with a deeper perspective.
This can lead people, such as myself, to investigate the value of a PhD itself and find that the career prospects, financial rewards, and even intellectual rewards, are difficult to align/justify.
I think that a reasonable conclusion is indeed that switching fields would generally be a good move, unless you know yourself to be an outstanding researcher within your field of expertise.
There is also the isolation factor. Many people do work in groups, and many people do work closely with their advisor. But, in the end, the student is expected to own some research themselves. That is the work that is the core of their thesis. Requiring the student to own some significant piece of work on their own contributes to the pressure people put on themselves, and the isolation they tend to feel.