Arena claims you can drop prisoners in it for an execution and an XP boost, but this isn't working.
Arena tooltip: "Allows the Beastmaster to train your Beasts or any intelligent minions you drop into the pit. Prisoners dropped in the pit will be swiftly executed, granting a large bounty of experience to all involved. [...]"
When you drop a prisoner in the pit and your minions kill it, it's not executed, just KO'd and dumped back in prison. Also, there is no large experience reward that I can see.