Here we go again. On the question of slide toppers it's pretty simple
Advantages
Keep rain sun and tree debris off slide roof. This helps avoid slide roof leaks and drawing debris and water into the rig when pulling in to depart without the need to go onto roof and sweep which is especially dangerous on a wet roof and if raining you will still be drawing in water if no topper. Also helps rig remain cooler in summer sun.
Disadvantages
Cost. Can be some flapping noise in large wind storms but rarely will rip unless already worn. I've only experienced this once on the Pacifica cal coast. Ocean front site. Wind rocking rig was far more bothersome than the flapping. No rips. Never heard flapping otherwise.