Thanks for the suggestion; we'll put that on the wishlist to consider as a new feature. BTW, this would only affect checkout pages that we manage: Free Checkout or card-based direct payments using PayPal Pro or Authorize.Net.
For other checkouts performed on a payment processor's site such as PayPal or Google Checkout, we take the email associated with the buyer's payment account (e.g., buyer's PayPal login email), since that should be a known-good address.