I may add that Sarute were a plus for a sword.
The Japanese army officers had to buy an army parade sword and a shin-gunto. It was the regulations at that time.
Navy officers had to buy a Kai-gunto, a navy parade sword and a navy dagger.
Of course during WWII, those regulations were may be less strictly enforced.
But anyway, ordering a sword at that time was a costly affair. Especially for Japanese officers which were not known to win a lot of money.
Every plus, like a gendai blade, thick seppas, pierced guard, nice sarute were kind of costly, so I assume that sarute made of fabric were less expensive that nice iron or copper ones.
The tactical choice to make less noise is also relevant. Moreover, fabric sarute seemed to be more resistant than nice ones which were held by little "nails" (I don't know how to qualify that part in english) of 2mm on each side.
Hope it helps.