Ok, the more I think about this the more curious I get!
There are a few US custom blank builders that produce extremely expensive Tournament "Pro" lure rods out of S2 Glass. They tend to be for Bass fishing where fish are "skull dragged" out of cover and to the boat in double quick time.
They use the glass because carbon rods can and do exploded when "hauling hogs" whilst the glass is much more forgiving.
The same goes for Catfish rods, saltwater Jig rods, heavy duty Jerkbait rods and so on.
So basically, the reason I am intriged is because generally S or S2 glass is used for rods that require brute strength, not finesse and delicacy.
Seems to go directly against all reasoning to use glass for a fly rod. It obviously works, so who am I to argue, but I am still intrigued