The title is pretty self explanatory. I think it'd be really nice if weapons which you can't sheath only took up one item slot. This helps make a lot of classes more flexible and realistic. Knights could actually have a longsword, shield, and lance, for example. Shield infantry could carry a pike without having to downgrade their shield.
With this system you could also add the "can't sheath" property to some of the biggest weapons while simultaneously reducing them to 1 slot, encouraging some additional creativity in equipment. Archers might bring weapons along but leave them on the ground until needed, for example. Someone with a flameburger[sic] or long maul would have to drop it in order to equip their shield when faced with archers. You could even raise more shields to 2 slots without breaking builds that use both shields and large weapons. (Doing it by weight instead of size strikes me as rather silly too.)