This isn't about malicious applications stealing your data. It's about misconfigured or buggy applications giving people more access to your computer than you intend.
Think about how many professionally developed websites fall prey to SQL injections, CSRF vulnerabilities, server exploits and the like. Nginx recently issued a warning about a big vulnerability in all but the most recent version. Github got owned by Egor Homakov using an old Rails exploit.
And those are professionally maintained. When your average "I'm not computer-savvy, but I can work Microsoft pretty well" user is the one setting up and maintaining the server, how long do you reckon it will be before somebody takes over his computer? My guess is that you're very optimistic if your answer uses hours as a unit.