I've been there. I've lost a cell phone and the 300 phone numbers that go with it. You don't plug someone's info into your email service and manage to lose it. Your Outlook and Gmail aren't sync'd together, nevermind your Yahoo and MSN emails.
So what's the solution?
The good news is that Plaxo has been around for a little while and has been able to develop. It has plugins for most webmail clients, Outlook, Thunderbird, Apple's mail, etc. Even better, if you're a Plaxo user and update your own contact information, it will send your new info to all of your friends. Remember when you had to send out an email or letter when you moved or changed your cell phone number? Exactly.
I'm guessing that 85% of the people reading this cannot see the direct value of this to their own situations. But think of it this way. You might not "need" it today, or at any forseeable point down the road. But that reality will change moving forward, as more and more people become entirely technomobile. You need to be able to have your information with you, and not just the first name/cell phone numbers you have in you flip phone. So why not take a preventative step now, so that when you lose your phone you already have things back up for you. And then one day when you have a smartphone, you'll be able to click a few buttons and sync all your contacts to your phone.
It just makes sense.