Zone or not I think we all agree that dwarves need to stop being afraid of creatures that they shouldn't be afraid of because they can't get to them. I am actually thinking what GreatWyrmGold said is best now because then siege operators won't flee their posts.
That'd bring about issues with dwarves and creatures/enemies with ranged attacks though. Having a zone for it would also give more control over when you'd want dwarves to express this particular behaviour and avoid having to mess more than necessary with pathfinding and normal behaviour, possily introducing more bugs in the process. A zone would be much easier to control, and you could also much easier implement a "enjoyed watching the arena lately" thought, tying it to combat taking place in the zone rather than having to set up a bunch of parameters for what would count as arena combat.
As Xenxe noted, dwarves need to be less afraid of critters that can't harm them in general. Today's not the type to put in half-done fixes.