I'd speculate that the Noldorian blades glow because of artistry involved in
their making that bear the mark or contain some traces of the Secondary World
of Aman. The glow is similar to the aura given off by Glorfindel in
LOTR. It shines even brighter in the presence of negative engergy, the
creations of Morgoth, because the contast between the light and the darkness
brings out its inner properties.
In The Hobbit alone, though, having a sword serve as a Goblin early detection
device is another one of those magical elements from myth and fairy tale that
helps make an exciting
story.
