The reason dust trails were removed is they were buggy. If you swooped straight down while at the ground, the trails would endlessly appear in the same spot until the swoop ended, which caused the fps to drop to 2 at times. This was realized when the bots were implemented. Now I know for a fact that, that can be easily be fixed, but I believe the ESF team thinks that fixing the swoop trails isn't a high priority to worry about.
Now yes, the swoop trails were indeed interesting to watch. They caused alot of lag, and most people didn't know how to turn them off.
If they were to be re-added, it should be defaulted as off, that way if people want to have them on, they can turn them on from the options menu, or console.