I tend to agree....buying a top quality knife without a sheath, is sort of like buying a luxury car without wheels/tires.
Whether the sheath is created by the Maker, or by an outside source, is up to each Maker, however, it bothers me when I see makers selling knives, especially higher end hunters and bowies, and a sheath isn't included. I know that some maker's opinion is that they are KNIFE makers, and not sheath makers, and that's OK, but I feel that a knife offered without a sheath, or other storage/display device is leaving something out.
I personally hate making sheaths, and for a long time looked for someone to do it for me. For one reason or another none of the prospects were acceptable to me, so I bit the bullet and learned how to do it myself. Often times now, customers comment on how nice my sheaths are, and in some cases they have told me that the decision to purchase, between one of my knives and another maker's, was the sheath.
If the knife warrants a sheath, I personally think that the maker should provide one with the knife. Of course there are other possibilities such as display stands, boxes with glass lids, etc. And to be completely clear, I don't think folders need to be offered with a sheath included.