Banking has turned into a computer based exchange and computers are fallible and they are also hackable. People should most definitely be concerned about security with the banking system relying on these computers as much as they are. If satellites fail or the system is hacked, it is unlikely that you will have access to your money.
Yes, we should be worried about Windows security concerns in ATMs, because there have been problems. People can add phishing software to the ATMs in order to glean information about people's private accounts. ATMs are insecure, and anyone can tamper with the software programming in order to cheat from someone else.
Yes we should be, lets use the hacking of Target as an example millions of people were affected. If there is no security in the ATMS in America your money is not going to be safe in a bank anymore and you will more than likely never be able to recover that money and the banks would loose a lot of the money.
The Windows operating system is capable of being hacked, or having a security breach, so we should be concerned. However, ATMs are so ubiquitous and used constantly by everyone, that I don't think people really do have much concern. It's difficult to imagine people functioning from day to day without using ATMs, so it's possible they believe that the benefits outweigh the risks.