Why does hack time fail?

Google are doing it, so it must be a great idea: hack time! In an innovative and enlightened company, everyone will be on board quickly. Reasoning along the line of, there needs to be some level of slack so people will have space to think and come up with new ideas, which will help current clients and will lead to new clients in the future. The constant display of thirst for new knowledge will also attract new employees and ultimately lead to a real engineering culture. »