Hi ChrissixD
In our opinion, it's more frequent to want to buy items to have something in stock (at least have 1 in stock) than adding items to just cover backorders. Just that.
If you have -4 and buy 4 from your supplier, the product will continue "out of stock". Perhaps this only makes sense for users that are not interested in having stock (like a dropshipping business).
Best Regards,
Salva.