My take on this is going to be short and sweet: You're crazy if you leave your wifi open. Here's what can or will happen if you don't secure your wifi:
- Your own download speeds suffer as neighbors' infected laptops find a new vector to spew spam and malware.
- You'll find your home IP address blacklisted and receiving spam complaints over bad stuff people send via your connection.
- The buck stops with you. Your ISP can trace it as far as you and no further. This means that if somebody uses your wifi network to send spam, or traffic in kiddie porn, you're the one whose door the feds or the FTC are going to knock on.
- Running a mail server? You'll get blacklisted due to all of the above.
It might be really neat to leave your car unlocked, with the keys inside, so your neighbors can borrow it as needed. But, is it wise? C'mon, people!