Part of it with me is the craftwork aspect; while you CAN make wooden wands that are ornate and aesthetically pleasing, any fool can make a wooden wand (as has been exhibited by the number of teen- and tween-oriented Wicca materials). All that you really need is a length of wood, and, if done properly, a ritual or two. Glass wands, however... to make a glass wand, you actually need skill. The concept of the metal core appeals to me, as well: the idea that something so fragile can have a core of something so strong.

Plus, like crystals, glass has a lot of nifty focusing capabilities that wood just doesn't have. ;)

It might be interesting, actually, to see a glass wand with a core of wood AND metal -- banded, perhaps, or somehow carefully intertwined. The Living and Unliving Earth in a shell of the four elements.