Allocation of workload effort to job roles

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.