So you’re self-employed and someone sends you a spoofed e-mail with work in it. How do you know it’s spoofed? You don’t. Do you open the attached file from your client? Sure, why not. Could it contain custom-written code? Oh yeah. Think your antivirus will catch it? Think again.
Oops. Too late. You’re already toast.
Think that only big companies have to be concerned? Bwah ha ha ha.