When I am initially setting up a project I work out the WBS to define the tasks and then I estimate the effort required for each job role. Managing resources across multiple projects should be done at the job role level - not the individual resource level. Later on if the project gets approved the job roles are then assigned to a team member for implementation. At present job roles seem to have evolved from groups. Instead a job role should be treated as a proxy for an actual resource and as such should be allocated workload estimates.