You could be out shopping and a criminal hacker with a card skimming device is sending out radio signals that detect your card. In an instant, they have copied all the details from your contactless card into their device. You can read more at https://en.wikipedia.org/wiki/Wireless_identity_theft
The card skimming hackers, criminals too lazy to do anything productive with their skills, make a clone of your card (or just a microchip in a wallet, no need to even make a card at all) which can then be used multiple times without difficulty for contactless spending on your account you pay but they get the goods you paid for.
Criminal Hackers Can Get Your Card Details on Your Travel Destination Photo Gallery
The bank might reimburse you, if you can prove they were fraudulent transactions (not an easy task), but even if the bank gives you money back, in the meantime you lose the available credit you had on your credit card or your account goes overdrawn and you lose the ability to spend anything.
Or the bank compensates you, but due to losing so much money due to fraud, they will find ways to get that back from their customers through extra or higher bank charges, which means you pay more in the long run.
You end up losing money, even if only temporarily. The money taken from your contactless cards can quickly add up so that you then get blacklisted for credit, and cannot get loans or credit anywhere for a time until it is sorted out (maybe it never gets fully sorted out).
An article at http://www.wired.com/2014/11/chip-n-pin-foreign-currency-vulnerability/ states “Flaw in New ‘Secure’ Credit Cards Would Let Hackers Steal $1M Per Card” you do not want hackers to spend a million dollars on your cards, do you?