Most of the control panels or billing systems use either a username/password pair or an email/password pair as login credentials.

Are there any specific reasons for not using domain Name/password pair on login forms instead? Wouldn't that be unique across clients as well?

I'd be surprised if I had a single client that only has the one domain hosted on our network. Even users with just one site will have the same domain name bought on a number of different TLDs. That's why we've never used it.. We have some clients that have more than one "contact" on the account. This means more than one person can login to an account without sharing the password. That wouldn't really work well with domain names as the UID.

It would work as an identifier for an account no problem though, as long as .s are allowed of course.

Though, an email address should be unique enough really.

Thanks for the reply Xeentech.

Yes, I was thinking of it from the point of logging into a single acount, like a billing system, eg using example.com. The ". shouldn't be a problem as using an email address to login also has to allow ".".

I was thinking a domain name would less likely to be changed than an email address.

But I see the point about multiple contacts / logins for the single account.

I guess for multiple contacts using email addresses, the system would have to keep track of which is the master account and which is the sub-accounts. If so, I guess a similar thing could be done if using a domain name, eg a.example.com, b.example.com etc for multiple users.