Configure each activity in the section does'nt resolve all section access control issues: for example I can't restrict user's permission to add activity to particular sections. (For example let imagine the course with a section for student created activities.)
As for the CONTEXT_ACTIVITY_ELEMENT, I think that it must not be centrally implemented. Instead it allows activity module to implement it's own schema of access contol to its elements. Some activity modules can greatly benefit from this, especially wiki (most wiki engines have ACLs now).