I’m a very strong advocate for slack. I believe introducing slack as a policy improves the overall quality produced during productive hours compared with the traditional approach of applying constant pressure. Just like pair programming, it is actually very difficult to get managers to understand the real benefits of having ‘slack’.
For me, slack is just another way to improve the transparency of an agile project and is also the ideal vehicle for introducing diversity, continuous learning and continuous improvement. Given slack, developers will naturally use this to either improve their own networks, improve their understanding on a topic they found difficult or explore new technologies. Any of these will ultimately add significant value to your project.