Well, it makes it more difficult for the NSA or anyone else to read my mail without asking me or breaking into my house. They can still do it, but it means they have to try that little bit harder and use another method. It's all where I get at it, I can easily keep backups and I don't get bothered with adverts if I read it using webmail.
I use a Sheevaplug for my mail server too, although mail is stored on an external USB hard drive, I wouldn't trust the SD card for that (had a few of those go down due to bit rot).
Of course, it's fun to learn how to set it all up and make it work, knowing how to run such things can be useful on occasion. I run two DHCP servers and two DNS servers on my home network too, in a master/slave arrangement so if one goes down, everything falls back to the other one. Sheevaplugs are good for this, extra resources, low power consumption.