in my experience the sheath is a decent indicator of the quality of the knife. if the sheath is of high quality, odds are the knife is at least decent, whereas if the sheath looks ready to crumble on the table, odds are the knife itself isn't worth owning.
this doesn't apply to antiques obviously, though an old sheath in good condition would indicate that the knife was well cared for, and therefore less likely to have fatal corrosion under the handle.
none of this is law, but it is worth paying attention to.