Two reasons to want that step:
1. The back of a SD is really high. I used to just step up from one of the rear wheels if the tailgate was down and I couldn't use the rear bumper, but now I have 20 inch tires and a few more years and it's a damned big step up. I thought that tailgate step was completely silly-assed, especially switching from GM Duramaxes (and GM's design philosophy doesn't protect the rear clearance like Ford does, but does gain it a much more accessible box height). Now that I'm driving this new truck, I actually use the step.
2. The brackets for the step add a lot of reinforcement under the cover for the tailgate. It's a lot heavier, but it also has a lot more strength. I've also been dragging pallets and forklift blades right over the composite tailgate inside liner, and haven't had any kind of marking. The stuff is tough. I am loading items into the back all the time, and at times having to shovel or sweep out junk from the bed (and that stuff likes to get into the cracks around the step). But if you just have a small sturdy plastic tarp and cover the tailgate (put it down before you load your bed, and when you later drop your tailgate it just covers the whole tailgate and also keeps junk from falling into the gap between bed and tailgate), it works just fine.