Ideally, a team member should be able to fill roles based on the alignment of role to their talent (i.e. what they can contribute to the organization).
This becomes an issue if a role requires a certain amount of energy (i.e. time) from a role-filler to meet the needs of the organization, then there may be a need to enact a policy to limit the role-filler from spreading their time (aka resource) too thin across the organization.
We have a policy in place that defines what constitutes a core role (think time and energy) and limits the number of core roles a team member can hold in the organization. This is an important distinction (for us anyway) because the core roles are linked to budget and must, therefore, be monitored/controlled by the circle Lead Links who are responsible for resource allocation.