You might have an email address from college that isn't the most professional.
Maybe you couldn't get the address you wanted?
Maybe you just want to have your vanity address on all the email services?
You wouldn't want someone to impersonate your Yahoo address on AOL?
My old Yahoo account (10+ years old) is plagued with spam, lots of spam. The filter doesn't seem to work nearly as well as my Gmail or AOL account. Over the past few years I've been moving most of my friends and company registered email addresses off my Yahoo account simply for the reason I get between 4 and 10 SPAM emails a day. It's a waste of my time to clean out these unwanted emails. My iPhone is constantly dinging as get this SPAM. The other day I tried registering my normal email address on Yahoo and it was available, it was time to retire my old account.
Today I took my account out back and erased it.
Sure there is still some concern that I'll miss an email from someone, but with all the different social networking solutions I'm sure they'll find me again. Whether the Spammers will is a whole different story.