I tend to use a desktop or laptop computer for gaming preparation and/or execution. And it was only recently I familiarized myself with QR codes because my annual insurance bill could not be paid online due to ongoing web site technical difficulties. I decided to try scanning the code on my physical paper statement in lieu of using the phone or postal mail. [This company has not yet gone paperless.] I grabbed a free iPhone app and it worked fine for scanning the code and reaching the site. It is still not my preference to type, expand and scroll around in a smartphone-sized computing UI.
Also, I personally am intrigued when books--and I have scads upon scads of them in all genres--contain references to online resources. For favorite resources, I will carefully check every interesting link and bookmark it according to relevance. Complex URL's are annoying to type from physical books. I also understand that sometimes sites are discontinued after a period of time especially if they are tied specifically to a single book title.
Some of my favorite D&D WotC and 3rd party books had electronic content available on their sites: errata, modules and other special content. I would usually check the vendor's site regardless of whether there was a reference to such content in the physical book.
Having said all of that: I think it would be useful to have a QR code in addition to a spelled-out URL. This would indicate a type of techie forward-thinking and could be especially appealing when a portion of the content is sci-fi themed.
I do not have the figures on the cost-benefit analysis, so beyond a certain cost the utility to the majority of users may not make sense. But within certain ranges, this product would join the ranks of those on the forward edge of scannable code adoption.
Practically speaking, I could see users at the gaming table with the physical book who scan the code to get to some online randomizer or dice-rolling util, a wiki or other searchable resource, forums, etc. They might bookmark those links for later/repeated use, but the initial scan to reach them would be a positive experience and memory for this vendor and product.
Linking specific topics via codes is something that sophisticated textbooks do using URL's at least. For some customers/users, being able to scan to reach a specific section on a site on demand and according to need would be a competitive advantage for a highly-referenced and re-usable print product with electronic (updatable!) support.
My friends that game with multiple structured-play groups state that "most" people use mobile phones or tablets at the sessions for dice rollers, character managers, supplemental PDF resources, and so on. It would seem that adding an electronically usable code to a physical book is not a far-fetched proposition at all!