Ronnie, not all digital products require only "1" quantity. Some merchants sell software or phone cards and buyers want more than 1 codes sometime.
Even for ebooks, if a corporate is placing an order to buy an ebook for their organization, they buy multiple copies.
Also, quantity in our cart only increases when buyer manually enters the number and presses the update button. If buyer simply presses the ADD TO CART button twice, we don't increase the quantity .. so there is no chance of a buyer "accidentaly" buying 2 of your product.