[Stepping on my soap box]
For me, the value lies in linking the schedule to the estimate/budget. If your estimate/budget is not organized by project WBS, there is little additional value over activity codes - except that P6 seems to really like using a WBS.
No suggestion that any schedule not be fully integrated - resource loaded - with Estimating and Accounting. In other words, IMHO, all schedules should be resource loaded and therefore rely on a fully developed WBS - use activity codes whenever the need arises. It is not an either/or, but rather what is appropriate.
All schedules should be resource loaded from a source external to the planner/scheduler and this source be cited in the Schedule Basis.
[now stepping down from my soap box]