Extra domains make the network more complex to administer. In the past extra domains were created because of the limitations in creating a large number of objects in active directory and there was only one password policy per domain. Now active directory can scale to millions of objects and it has more than one password policy per domain. Before you create a child domain, ask the following question.
Why Do You Create a Child Domain?
Companies are often sold and merged. Sometimes employees are transfered within two same companies. As a result, employees have reassigned locations that are part of different domains. when this happen, you may also need to migrate AD objects between domains on daily basis.