Would you place your home phone number in a place where millions of people can see it? Well, you maybe exposing your private email address to millions of people every time you send an email, every time you download a file from a FTP site, every time you subscribe to a mailing list or a web page notification, every time you submit forms, every time ... (and the list goes on) :)
So what's the downside? One day you might come home and your email box may have more junk mail than your regular mail box. While it's difficult to completely stop your private email address being exposed to strangers, you can certainly try.
One way to handle this problem is to get a secondary email box and use it to send mail to "public" places, while using your primary email address to communicate with your friends and family. If your secondary mailbox starts to receive too many junk mail, you can simply close it or change it's name.