Please feel free to correct me if my theories are wrong:
majority of collectors
If a card collector does just that...collect cards...how does it matter if the back's one-way or two? I certainly don't mind.
practically every card player
I can see deceptive gamblers and magicians using a one-way back in their respective fields. If it's an obvious one-way, I wouldn't appreciate it, as would the next person. However, if it's something discrete like this, or the Karnival decks, I see many people who can reap the benefits.
First answer: "collectors" covers the whole gamut of card people: magicians, collectors, flourishers, poker players, etc. Of those groups, magicians and poker/card players would be the largest. Poker players wouldn't touch a one-way back with a ten-foot pole, out of concern that some card sharp will use them to their disadvantage. Magicians tend to stay away from them as well, for similar reasons. You did catch that "and practically every card player" part, right - the part you edited out of the quote? If it's Go Fish or War, no one cares, but for any serious game (except, bizarrely, bridge), players prefer two-way backs. If you don't mind, good for you and good for Lance, assuming you were his only customer... But as stated, that image is only a preview, not the finished back.
BTW: while some of the early one-way backs from Karnival were more subtle, some are ridiculously obvious. Only decks of theirs where I haven't spotted a one-way mark (yet) are Karnival Earthtone9 and both editions of the Karnival Death Heads.