How do you feel about vending stamps on vintage Topps cards?

dude

I little known fact is that Topps would stamp the back of a very small percentage of vending cards so that when someone received one of these cards from the vending machine, they were awarded a bonus prize from the shop owner. I've had the opportunity in years past to go through vending runs of '68 and '69 Topps baseball cards and have one in my collection: a '68 Topps Len Gabrielson PSA 9 card. (Yes, PSA does grade them, and the doesn't put a MK qualifier on the grade either).

So my question for all of you is this: does the stamp mark improve the appeal or collectibility of the card or does it detract or