Attached is a pic of frame piece of mine. I cut it off the end of crossbar I was repurposing. The bar (and old frame) was bought new in 2002 and lived on my hyside until I sold it last year. The visible wear is actually a flat spot on the tubing... you can feel it. All that wear occurred between frame and rubber (+added sand, silt, road grit etc. I'm sure). over 12 years of recreational use. The rig had many thousands of river miles and many, many thousands of road miles on the trailer.
In my experience time and grit will combine to wear off any coating, it's just gonna happen. You need to decide if the result bothers you or not...I personally don't mind a few black marks on the boat, makes me think I'm getting my money's worth from it... But notice that the rest of the tube is still well covered with anno. To me that's important, no maintenance, pealing paint, chipping powder coat, just a few dings and otherwise solid coating.
Why is the coating important? I have two frame pieces that aren't anodized and the rest that is (probably 100 lineal feet of NRS sourced tubing) and the non-anodized pieces make your hands black if you touch them. They're really annoying! Gary typically suggests to paint with clear coat (krylon) and I'm sure that works fine, but I have never had great success with aluminum and common spray paint (it really needs to be etched, primed then painted) and fear it will just start chipping/pealing off. I may spend a few bucks this winter and get them powder coated since there's a place nearby. I'd get them anodized but no one does that nearby.
To me it makes sense to buy NRS tubing because I don't have a large distributer nearby and I want the parts coated with something. Plus having a frame anodized or powder coated after the fact will be expensive (my huge frame would be really expensive). I'd love to support the small local business that sells USA made products, but in reality Pacific Steel (only local source for AL) is much larger than NRS and I'd bet their AL comes from a similar (possibly the same) source as NRS.