[Chuck Burrows]

Main Entry: scab?bard
Pronunciation: 'ska-b&rd
Function: noun
Etymology: Middle English scaubert, from Anglo-French escaubers
: a sheath for a sword, dagger, or bayonet

Main Entry: sheath
Pronunciation: 'shEth
Function: noun
Inflected Form(s): plural sheaths /'shE[th]z, 'shEths/
Etymology: Middle English shethe, from Old English scEath; akin to Old High German sceida sheath and perhaps to Latin scindere to split
1 : a case for a blade (as of a knife)

As you can see by the MW Dictionary scabbard is essentially just another term for a sheath although perhaps more specific, but a dagger is a knife so..... Personally I call a case for a a knife interchangeably a sheath or a scabbard, but more often than not call a case for a sword a scabbard.

Hardness really doesn't play a factor in my mind since leather can be formed as hard as wood or kydex by using the ancient cuir bouilli method and I make most of my knife sheaths stiff if not hard. Another instance - WWI and WWII bayonet "scabbards" were all made of leather and are all hard/stiff.

On another note the verb form is always sheathe - the verb form of sheath - you never hear of someone "scabbarding" their sword - they sheathe it.