Well, from what I've read from the wiki, elves usually keep the vhenadahl simply because at some point it was important in their culture. Now [b]why[/b] it's important is lost to the time, so for the most part they just keep it around for as long as it is practical. Weapons can be made from the vhenadahl however, as some weapons in Awakening are. Besides that it's not that they're just going to cut off a branch for Myrn, they're going to give her one made by an Elf a while back, after she does some quests for them.