Currently, we can give a client a gobal dashboard access user account for all of their sites, which is important. However, in a more distributed environment, it would be helpful to give other site admins credentials for a particular site or sites.
Since user credentials must be unique, I can already see a problem if a particular admin has responsibility for more than one site. So, if the suggestion makes sense to implement, it would probably be best to allow a client user to be setup, then associate that user account with one or more sites via a check box list.
I believe the above would allow complete flexibility to meet the requirements of any particular client, since it would allow multiple client users access to whichever sites are appropriate for them.