I think you are asking about the solid, plastic+metal cover that mounts at the top edge of the slide, not on the RV box, right? Such that when you are driving down the road the wind cannot catch the rolled-up topper fabric and unravel it as you drive 62 mph down the highway, right?
I would get them. At the very least, it's going to save you frustration and keep your toppers from whipping and fraying in the wind.
Ken & Deb (WIT Club, FMCA, SKP, Harvest Hosts, and grateful volunteers with SOWERs)
2005 Winnebago Vectra 40FD w/400ISL and Allison 3000MH on Freightliner's Evolution chassis, Onan QD7500, 1000w Renogy solar, BlueOx tow gear, and 2015 Cherokee TrailHawk toad.