But we can't know if the costs are massive.

We should be able to make an educated guess. If not maybe we can do a small spike and get a better estimate of what the costs might be. We should also be able to pull back if the costs are to high.

Obviously if there is a major impact you don't do it.

Plus, it's One More Thing To Think About. Distracting the worker/boss/department from their real job has costs.

True. It's often less costly than many people think though. And the cost of making employees unhappy, not allowing them to explore new areas of interest and preventing them from doing anything that doesn't exactly match their current job description is often ignored.