Just a quick observation from a customers side ...
When the quantity box is removed and only the Add to Cart button is there, I would click it to add the product to my cart. Okay no problem. The confusion part comes in when viewing the Shopping Cart. I can see the product I added and the quantity of 1. Also, next to the quantity is a picture
(button_update_cart.gif).
When you put your mouse over the picture it says: "Change your quantity by highlighting the number in the box, correcting the quantity a..."
Well that can be very confusing and frustrating. If you decide you want more of that product then according to the instructions you just received by the mouse-over text on that update cart button, you can just highlight the quantity and change it ... but you can't. It's impossible to change the quantity from within the shopping cart once the Quantity Box has been removed. As a customer I would get really frustrated and possibly just leave.
Maybe some text in the shopping cart that says if you decide you want more of a particular item, click the item in the Shopping Cart to return to the item details page, then click Add to Cart again.
Ideally it would be nice to have the Quantity Box removed from the product listing page yet still have it in the Shopping Cart. Only way I can think of doing this is to manually edit the product listing page code and remove it there. Leave it enabled in the Admin area so it still shows on the Shopping Cart. If that was possible it would be nice.
Find a way to remove the button_update_cart.gif from the Shopping Cart if you don't have Quantity Boxes.
Just my observation and only written to give another perspective on this.
Steve
Zen cart version: 1.3.9h