There are parts of the world where people don't use email like we do. Here we tend to keep few email addresses and we typically don't mind signing up using a primary email address. In other places, they trust no one regarding email so they use throwaway email addresses. This makes it hard when someone comes back to the services as they don't know which email they signed up with.

If you a like Twitter and trying to cross-reference people's email/phone from others address book to get people to follow each other, the throwaway email accounts are worthless compared to more stable phone numbers.