To most, this seems like a lifesaver, especially if your battery is almost completely drained.
Unfortunately, hackers have hooked tiny computers into some of these USB ports, so when you plug your phone in, they can install malicious programs on your phone. These programs report back personally identifiable information that thieves can use to commit identity theft.
Alternately, thieves can use the connection to your phone to look through your phone's contents, stealing browser history data – including passwords. It's called "Juice Jacking," and it can take as little as three minutes for them to steal your personal information.