For the longest time now I've been filtering my spam, but still storing it into folders. The theory was I'd examine those folders and make sure nothing valuable slipped through. Of course that never happened, and instead I'd just go delete megabytes of crap every few days.
Yesterday I changed my mail setup to just automatically delete spam. And worms. And bounces from spam forged in my name. No longer storing it, just delete it. The psychological difference is enormous. My mailbox feels much lighter. I no longer have the pressure of "1000 unread messages in folder spam". Then again if you need to mail me a Windows executable I won't ever see it.
I archive absolutely all my mail, including spam. But I never even look at the archive files, so it's no big deal.
Last I checked, ⅔ of my mail was spam.