Prior to the proliferation of Bluetooth, in order to skim a card, the crook needed physical access to the card reader to plant the skimmer, and physical access each time they wanted to retrieve data. Now that Bluetooth is common, they only need physical access once to plant the skimmer. As long as no one detects the skimmer, the crook can reap the rewards without any additional risk of getting caught.
This is also good news for consumers, as it allows apps like the above mentioned Skimmer Scanner which looks for a signature used by many skimmers to detect them. I know I’ll be using it every time I pull up to a pump or ATM from now on.