your subordinates don't have to understand why everything gets done, they just have to be onboard with accomplishing the goal. i would focus more on teambuilding, and less on pointing out that things are dumb. you might not even know the real reason why it was assigned to you in the first place. maybe your boss just wants to see if you can handle an objective in a timely manner. acknowledging its dumb to your staff does nothing except erode confidence in the team as a whole. find someone good at the task, make them a team lead, and assign a few people you want to develop into that role. in the end, this approach will help you, and everyone else out far more than simply saying 'we have to do this thing, but it's dumb.'

being an effective manager is about more than just honesty. you have to find a way to be honest, but also genuine, and stay true to your company's ideals.