The e-mail address is more important than other fields both because it can be used to verify identity and because it allows a website to send marketing e-mails (which is often the purpose of getting people to sign up.)
Then again, you can do a lot of mistakes when typing your shipping address and it will still get through. It is also easier to spot mistakes since a shipping address contain regular words which isn't as common with email addresses.
One char wrong and your email is completely worthless, and with it your only real link with the service is cut off - often with no way of fixing it.
