For now I’ll say that there are theoretical reasons to think small amounts of some types of fiber may be beneficial and are well within the realm of our evolutionary history, but the idea that fiber supplements or the large amounts that come with a vegetarian diet are necessary to optimum health is at best unproven. Massive amounts of fiber are not likely to be any more a requirement than massive amounts of starch or saturated fat. Good? Maybe. The more the better? Very implausible. We're not gorillas.