tradeable = MU for unranked players
Non-tradeable = prevents socs grinding and passing them to a single player who sits 24/7 in the hub
while I see where you are coming from with this I don't think it is a good solution. I've been on this mission for years now but skyshatters and lava Golems are out of my range. If i'm going to want to hunt those i'm going to need a team. making the vouchers bound would just ensure more rewards for strong players and leave weaker ones with nothing.
Sorry i'm not skilling up as fast as others but i'm active and I shouldn't be punished just cause of my skills