Our staff that actually pick and ship orders much prefer to use the Order Summaries as the primary document when processing an order. This is because it has the most necessary information and because they can usually fit multiple orders per printed page.
However, they point out that the Item Quantity is across the page from Item SKU, which makes it easy to get mixed between multiple line items, especially when working quickly.
Adding Line Item separators would solve them quite well. The lines could be dotted or a lighter grey if neccessary, to not confuse a line-item-separator with an order-separator
I realize we could create a custom packing slip to look very similar to what we want, but switching back and forth every day between an "order processing" packing slip and our many different customer specific templates sounds like a recipe for disaster.