What is offline domain joining, how effective in an organisation?

Offline domain join is a new process that computers that run Windows® 7 or Windows Server® 2008 R2 can use to join a domain without contacting a domain controller. This makes it possible to join computers to a domain in locations where there is no connectivity to a corporate network.

For example, an organization might need to deploy many virtual machines in a datacenter. Offline domain join makes it possible for the virtual machines to be joined to the domain when they initially start after the installation of the operating system. No additional restart is required to complete the domain join. This can significantly reduce the overall time that is required for wide-scale virtual-machine deployments.

A domain join establishes a trust relationship between a computer running a Windows operating system and an Active Directory® domain. This operation requires state changes to Active Directory Domain Services (AD DS) and state changes on the computer that is joining the domain. To complete a domain join in the past using previous Windows® operating systems, the computer that joined the domain had to be running and it had to have network connectivity to contact a domain controller. Offline domain join provides the following advantages over the previous requirements:

• The Active Directory state changes are completed without any network traffic to the computer. 
• The computer state changes are completed without any network traffic to a domain controller. 
• Each set of changes can be completed at a different time. 

Fore more info read this article 

http://technet.microsoft.com/en-us/library/offline-domain-join-djoin-step-by-step(v=ws.10).aspx

it is a method to join a computer to a domain in case it has no connection with DC, you can acheive this by usiing a command that will store a config file then you import the file to AD by same command as well.

Can be used with PCs imaging and with portable PCs