I can see the confusion that noi.ng might be experiencing.

Using my test site of jeandret.com and going to all products, once you scroll either of the Bug's Life items to the top of the screen, the Add Selected to Cart is not visible either at the top or the bottom.

One could assume (I know) that, when the quantity is changed from 0 to some positive number, the Add: would become an active submit button. Thus not having to scroll up or down to find the Add Selected to Cart.

Please note that this is not something new or that only happens with Bootstrap. It's been around a while and doesn't become a problem if all items have attributes, or are on special, or anything else that disables buying the item on the all products page.

Might be a 1.5.8 consideration.