Why do I have the feeling again, that nobody is taking me seriously? 
Okay, maybe I just didn't express myself properly. There is a reason why Curgen's Hammer is part of the Master Quest. It has significance to the plot. You learn about the Hammer (it's really a maul and not a hammer) and it's original owner and creator and why you need it to open the gate. When you finally acquire it, you know, that you not only have a powerful artifact in your hands, but also a piece of Elementals history.
Not so much with the Arena quest. The Hammer just happens to be the grand prize for defeating a couple Obsidian Golems. Why? No reason, it just is. If you don't know who Curgen was, you probably won't care about the historical significance of the weapon, only about its performance in combat.
In my first beta game, I happened upon the Arena and beat the Obsidian Golems. I was awed by the weapon, because I read the Hiergamemnon, that came with the LE of E:WoM, and knew who Curgen was. It was like having Grond, the mace of Morgoth (Sauron's old master), in my hand. Well, the Overpower ability of the Hammer might also have something to do with it. It really is a devastating weapon against armies. Later I went on the Master Quest and when I learned, that I needed the Hammer to open the Black Gate, I thought: "I already have Curgen's Hammer. Can I go back to the Gate now?...No? Fine. Whatever." This pretty much robbed the Hammer of any significance and felt just weird considering the plot.
The possibility of getting the Hammer twice made no sense to me back then and it still doesn't. What makes something unique is the fact, that there is only one like it in the world. If there are two, it is no longer unique. In this case, it makes no sense, that you can get Curgen's Hammer as a prize at the Arena. It could have been any other powerful weapon and it would have made no difference.
