I really want to defend bungies decision, I really do but if arena mode is something like- just mindless waves of enemies in a static area, it'll be extremely boring and repetitive. That being said, if its anything unique and not just the run of the mill mindless horde mode it will be fun. For me, the mode will be badass if the environment is huge and have epic boss battles that require teamwork as well as individual skill. Heck, I even want there to be some competition between the players themselves- not like a pvp competition though, something like the three guardians spawn together and have to take a separate path and survive a wave of enemies on their own for a while before meeting up on a vantage point etc. That will promote competition between players, who died first? Who reached the meet up point the fastest. Upon meeting, there could be team objectives and of course theres always the "who got the most kills". ANYTHING to make it interesting, just not a simple mindless horde mode in a limited confined space.