Use your nym to log in. Using your nym makes surfing the web private and secure.
When you use a nym you conceal your real identity. Plus you control who can send you email -- without ever giving away your real email address.
Customers shouldn't have to leave your site right at the moment they are about to sign up. Account registration should have a minimal impact on your conversion rates.
You need to get an email address and get users started with as little friction as possible. Nyms give you a single sign-on option that your customers will understand and won't hesitate to use -- and you get an email address that works.