Sure, you have nothing to hide, but encrypting your e-mail can help others… no matter how unimportant your messages are!
Who will be harmed the most now the NSA is convinced that E-mail encryption is suspicious enough for them to store it and try and crack it? Criminals? Terrorists? More likely it is going to be journalists, whistleblowers and other dissidents. Just look at how both the US and UK governments have been treating people like Glenn Greenwald and his partner, David Miranda, Laura Poitras and Jacob Appelbaum, or the OWS movement and other peaceful, dissident groups. Just see how whistleblowers are either on the run (Edward Snowden) or behind bars (Chelsea Manning).
So, if you “have nothing to hide” and all your e-mails are along the lines of “I’ll be late for dinner!” then you should definitely use e-mail encryption… for the sake of others. Every innocuous encrypted e-mail the NSA or other organisations intercepts costs them time and resources, making it more likely that the encrypted e-mails that do matter — those sent between whistleblowers and journalists, or between dissidents organising a protest — can hide for longer among the other messages.
Encryption really isn’t that hard to set up on any computer, especially with great tools such as GPGtools for OS X, GPG4win on Windows or the Evolution Mail client on Linux (Gnome). Unfortunately, no user friendly solution for iOS exists (yet?), though there are some good-looking Apps available for Android.
Just sending an occasional protected e-mail here or there already helps, so be social and encrypt your mail.