I'm surprised no one has mentioned the quality of yarn qua yarn. The texture, hand, color, durability, etc.
Go to any big box craft store and look at the yarns available from huge yarn manufacturers, some great stuff, some extremely ugly color combinations and strange textures I would never want to live with for the duration of any project, let alone live with the finished item.
There must be a market for all this yarn I hate or it wouldn't take up so much shelf space. I know some of my favorite colors must not be universally popular too -- surely all of us has exercised willpower and declined an impulse purchase of a beautiful yarn only to think of it months later to find it has been discontinued.
So I don't really care if yarnmakers knit if they can contribute to the variety of colors and materials that are available to me as a knitter and crocheter.
How far back in the design process would you demand knitting skill? I demand only that pattern designers know how to do the craft they are designing for. I don't care if Martha Stewart knits or crochets any more than I care if the dyer or spinner or sheep farmer or the sheep know how to knit.