Why should you upgrade WordPress?
Code Poet ~ Locking Down WordPress
WordPress is constantly being updated to allow for functional improvements and fix issues that position your online business at risk. WordPress makes it easy to update your site version and plugins. It’s best to upgrade your WordPress installation as soon as a security fix is released. Otherwise, an older (read insecure) version could be the target of an attack.
WordPress is open source software which means the information is available to the public. Being so publicly appreciated (more than 25% of all websites) the targeting by shady persons or directly harmful “basterds” is done by automated processes. Online is a hacker heaven and an outdated site is a bullseye for an attack, because of a direct connection. It’s not you, it's your outdated WordPress site.
Out there, in the online there are also extremely nice people, who try to help by offering solutions to counter these problems. These are the security firms, who constantly analyse, compare and warn everybody when a clever intrusion is identified. However, even this is against your outdated WordPress site. Public notices, that a security issue was found or resolved makes those few unwanted individuals grow in numbers as they know the update process is not instant for all the websites and there is a window of opportunity a few clicks away.