Yep--the glass is what makes it last. Like you said, there are some all-wood bows that are fast, but the problem I have with them is durability/longevity. That's why even primitive bows were (are) backed with rawhide, sinew, etc. You can basically bend a fiberglass lam in half and not hurt it--very durable, very flexible, and (usually) very consistent.

There's even bows that have foam cores instead of wood--Olympic bows, I think--the foam sure isn't doing much work. Pretty sure you couldn't use foam cores on just any 'ol design though.

Chad