Like others are saying, with ducted you can control the distribution of cold air better.
We had a 24' non-ducted and hated the blast of cold air in the middle seating area. You could open/close the fore or aft vents in the A/C but that was it.
Now have a 28' ducted. The central A/C has three vents (aft and the sides) that can be closed, and there are three ducts that can be closed and rotated.
Because the A/C is further from the bed in the new 28' trailer, it's quieter when sleeping. And with the bedroom ducts, we get a gentle, adjustable stream of cool air. No blast.