I know that it is preferred that we don't add every element swizzle, only the particularly useful ones, and I find this one to be particularly useful, as I constantly find myself adding it to graphics related projects.
Since I will probably be adding SwiftNumerics to any future graphics projects, it would be nice to have this particular swizzle included.
It'd hard to draw a line whether a swizzling is useful or not.
Even simd2.xyxy or simd2.xxyy, or even color.rgba vs color.argb is not that far out of reach.
I think zero-extending or truncating another SIMD vector is a special class that could reasonably be given its own affordance. I would probably want it to be more general than just SIMD3 from SIMD4, however.