An aspect that has not yet been mentioned is that most parrots get extremely attached to specific husbandry staff and serious mental problems can arise in parrots if they are parted from the person they have bonded to. Scarlets especially will pluck themselves bare if a bonded keeper gets a new job elsewhere. They do not bounce back easily from this.
Aside from the above mention aggression to other birds (and guests if it is a walk thru) the main problem that The Living Planet faced when discussing whether to have macaws on exhibit is that they would spend the day dismantling the H-Vac systems. That's the biggest problem with displaying large parrots - they get bored so easily that anything within reach is fair game to destroy - plants, cage structures, children, other birds, etc.
I personally also hate the "parrot on a stick" approach and thankfully we will not go that route, though that is the easy out that a lot of zoos use.
Here is the best case scenario for displaying Macaws - Keep them in the back of house. Give them a LONG TERM DEDICATED STAFF. Work with them DAILY. Bring them out for shows or walk arounds or even fly arounds. Do not try to display them the way other birds are displayed. Or, if a place does want to display them in a large, free flight enclosure they will need to overcome the obstacles listed above. (Which is exactly why most zoos just say, "To hell with it, clip the damn birds' wings and stick 'em over the koi pond!")