http://openid.net/add-openid/add-getting-started/
Here are just a few of the benefits that come from allowing your users to register and login to your site with an OpenID that they already have.
Increase registration and conversion rates
Lengthy registration forms pose a major barrier to sign up and conversion for web users. OpenID simplifies the registration process by allowing users to [...]
read more from "It’s easy to begin accepting OpenID on your website."