I have considered upgrading Rainbow Boulevard to trunk in the past as well for the reasons you stated, but ultimately I would have to believe that primary is the best classification for it. My initial considerations were its high traffic volumes and that it could be used for getting from Route 95 to the 215 to Highway 160, but it's still primarily local traffic using it, so I decided against it. Even though Rainbow does see more traffic than Durango, Jones, and Decatur, they all have very similar roles in the road grid, and the alternative would be to drop the latter roads down to secondary, which seems a little under-classified in my opinion. I think primary should be fine for all major thoroughfares of this nature in the Las Vegas Valley granted that they are mostly used by and intended for local traffic.