I don’t know about you but I often find it difficult to remember usernames for different websites. If someone can’t remember their login name they’re more than likely going to go somewhere else.
Having your log in use an email address rather than a username solves this problem so easily.

Ah, the day of the unique email per participant is past I think. Even those who have one address are likely to change it over time and going back to every site you’ve registered it with is going to be painful.
The solution, really truly, is OpenID. Poblem there for me at present is I don’t get to exercise it very often, not enough sites are using it, and I forget the URL
Hmm good point Hamish. I don’t think there is an easy solution to this problem. I myself have a separate email address that I use for all site registrations and online newsletters etc.
As Hamish said email addresses are not solution as people can and do change them. OpenID seems like a great idea although there are downsides. There is an interesting podcast over at Technometria on identity that is worth listening to - http://www.windley.com/archives/2007/04/drummond_reed_on_xri_and_identity.shtml
They bring up some good points and talk about XRI. I’m not sure if it is _the_ solution but it looks promising.