Personally I think Organisation Units are key to any PSA solution. It is a new concept for CRM people and in my opinion appears as more of an ERP concept, however it is a key feature, especially when doing inter-company billing or needing to integrate with an ERP.
The basic functionality of Organisation Units requires that a bookable resource, quote, project, project contract and invoice be linked to ONE Organisation/Contracting Unit.
For global professional services firms that employ technical resources in different practices or business areas, how costs are attributed between these practices can differ. The Organizational Unit concept has been designed to help with this need. It is meant to provide a way to group a set of roles relevant to an division of a company that tracks its costs for filling these roles using a cost price list.