There are some important benefit for "Quick View" methods for example:
Reduces server load. Since the information on the product is already stored on the page — hidden behind the scene — the page does not get reloaded when the visitor enables this feature. It is simply taken from a hidden state and put into a visible state.
Expedites shopping. Visitors can quickly add products to their carts
without having to wait for the actual product page to load. Having the
ability to browse through a category page and read expanded product
details in the quick view without having to load the product page,
will shave crucial seconds off the shopping experience.
Discounts to MAP. Manufacturers sometimes require a “minimum
advertised price” policy that limits merchants in how they list
product prices below MAP on their websites. But MAP policies do not
typically apply to quick view modals. Thus, merchants can show
discounted — below a manufacturer’s minimum advertised price — product
prices on the quick view. Show discounted pricing, other details. You
can utilize the quick view feature to show discounted product prices
before the cart page.
You can find more here: http://www.practicalecommerce.com/articles/3310-Using-Quick-Views-to-Increase-Conversions or http://www.electricvine.com/blog/webdesignnj/index.php/2012/06/5-e-commerce-optimization-trends/
In terms of ROI i think it's not easy to track this action to the number of products sold because there are to many variables during the process of buying.
What you can try to do track using custom events or A/B testing to understand if the "add to cart button" is clickable on the product page or in the Quick view mode.