The philosphy behind the Lazy Registration pattern is well depicted in this TechCrunch description of BlueDot.

The key is that Blue Dot is useful to readers immediately without registering for an account. Readers who click on the Blue Dot link to save an article on a partner site see a small pop up box from which they are able to send that article by email to anyone. The article is saved in an account automatically created for them without registering for Blue Dot. A cookie on the browser associates the user with that account and an email is sent to introduce Blue Dot’s full feature set and direct them back to the Blue Dot site to read friends’ comments about the emailed item.