David Carrier, an evolutionary biologist at the University of Utah, believes our key advantage is the dexterity and configuration of our thumb, which folds over the second and third fingers as a buttress, concentrating the striking power and protecting the delicate hand bones. (Crucially, male index fingers are short relative to the ring fingers, so they fit snugly behind the bulge of muscle at the thumb"s base; in women, the second and fourth fingers are typically the same length.) In a recent paper in the journal Biological Reviews, Carrier speculated that the bones of the masculine face may have co-evolved with the thumb to be able to withstand a punch. "It"s not settled," he said in an interview, "but the evidence suggests that the male hand evolved to be a better club, while the female hand maximizes dexterity."
It really depends on what you wish to achieve. For example, what if you want to flip someone off? Would you use your thumb? That was just a humorous example. However, If you were missing everything but your thumb, it would be useless. Besides, thumbs are simply another finger, a tool of society. We do not need thumbs more than any other finger other than maybe the pinky or ring finger. I think that all fingers are equally important.