It won’t necessarily be for a one time usage. I will give you an example.
I sell computers to clients that are custom built. I also use custom built computers (with most of the parts that I sell, apart from one or two items that I would not likely sell to clients). So I have quite a few computers that are for company use, not for resale. Having the parts in the non-inventory section means that I can re-order any part again quickly, as I just go to the part in non inventory, copy the part code into the website and voila I can place my order.
Even for a one time order for some things its useful to have it in non-inventory items - for example if I ever need to buy another office chair like the one that I just bought, it is easy for me to find the part code in manager, but only if I have the entry in non-inventory items.
I think what I will do is just put something like NI for non-inventory items that are identical to inventory items for the moment.