I do not share logins across multiple sites. Why not allow a user to register on this site? Why is the UX.stackexchange.com registration ux so different from every other site?
UX.SE, and the other SE sites use technology called OpenID. By using OpenID you are not sharing Login Details across multiple sites, you are sharing one Identity across multiple site, but only sharing Login Details with one site.
Many sites these days let you Log-in with Facebook, or with a Google Account, or with a Microsoft Account. These are other ID Providers, very similar to OpenID.
My university maintains dozen of sites, including logons for External Journals and goverment resources. Even thought it doesn't have any control over the sites, I can log in there, because it redirects to my Univerity ID Provider (and there i enter my university username and password), who verifies that I am part of the uni, and thus am entitled to the benifits the uni has paid for.
You can use OpenID in many other places, for example Facebook accepts OpenId logon, I used to use a forum that accepted it. Even PayPal accepts OpenId. Why OpenID?
Consider a relationship between the two entities:
- Website: Identify a person for there interactions with the site, provide password management etc, prove who that person is (which may require collecting information like address etc.)
- User: Maximise own privacy by minimising amount of people who have access to my details, be able to be Identified on a site.
So the goals the Website and the User Share, is the desire for the User to be Identified.
Consider the ID Provider 3 Entity relationships.
- User proves Identity to ID Provider, giving identifying details.
- ID Provider: Provides password management etc to User, and Indentifies (vouches for the identity of) the User to Website.
- Website: asks ID Provider to Identify (vouch for identity of) Users.
The is much better.
The User Wins as:
- Only have to trust, the OpenID provider with my details, not every site.
- More secure than using same log on detail on every Website, as remove the ability for a website owner to test out my username and password combination on every site i visit.
- More convenient than having different log on details for every site, as won't forget them.
Website Wins as:
- Save money on technical effort of managing a password systems.
- Reduce legal obligations from various Privacy Laws, as don't have access to the details the User used to prove identity.
- Improved User experience as, the can get right itno the site without having to go through long registration form.
Now StackExchange operates its own OpenID provider, but you don't have to use it. For example my local computer club is my OpenID provider. They know who I am, and I trust them alot more with my details than I would trust any old website I have just run across.