I think Quickview as an independent function is a necessary evil in that it loads faster than a product detail page. As a necessary step in a purchase pathway, it holds less value, likewise in all ideal cases, the product detail would carry its full weight in converting a transaction.
Your true question of whether next/previous is useful or a gimmick - it's definitely not a gimmick as it inherently doesn't add value to a site. However, if a quick-view is involved choosing the next product or previous isn't bad as a browsing experience - the possible issue would occur after closing the modal and indicating which product has been perused/browsed and which ones haven't.