I've just learnt that Resource planner ignores logged hours against parent tasks.
Since we have a configuration that requires assigning users to both the parent and the child task, this enables users to log on both. That leads to serious distortions in utilization figures.
However if we want to use this - otherwise incredibly useful - tool, we should have the possibility to either disable logging hours on parents to make it calculated similarly to applying Planned hours (also to keep consistency it could be calculated) or the Resource planner should consider the parent tasks too for logged hours.
This would also mean disabling parent tasks to appear in Timesheets in a way they do now.