knowledge can be shared without losing anything. Literally, Harry could order Bob to share all his knowledge with Bonnie and she would become more powerful, and he would not become less powerful. Its not like energy.
Even if you accept that acquiring knowledge does not require effort to share, it does still have, at minimum, one cost: time. This doesn't count Harry's instinctual use of Demonreach's Intellectus to run through the woods, because in that instance the knowledge being transferred is closer to "how do I wiggle my fingers" than "what is string theory." Even using Demonreach's Intellectus to discover more complicated information requires questioning, focus, and thought.