Wrong. Well...maybe it works that way for some people.
I wandered into my LYS yesterday for an advertised yarn swap. I'd brought along two skeins of Debbie Bliss Cashmerino yarns (really pretty, but in a color I seldom use).
I walked out with five skeins of Shetland wool! It's not sock yarn, so I think it counts as Stash Enhancement. (I was encouraged to do this by the shop employee...she's an enabler even when not selling the stuff.)
They're the same weight, and might work together in a project. Good for simple fair isle, especially with a bit of white or cream or black to balance things out.
I can always rationalize yarn acquisitions