I do not think that a project should have a separate portion called "slack time". However, extra time should be added to various steps of the project that are expected to have delay. For example, why apply slack time to a portion of the project that the team is very confident can be finished in a shorter time? This time can be better used by giving it to another part of the project. This is all to be done in the effort to stay on schedule, and not over or under assign time to any given part. Slack time can be extremely beneficial, especially when the project is dependent on people outside of the team, such as suppliers, testing, and upper management. This precision in assigning time to different parts of the project is valuable information that teams can use for other projects. Additionally, "hiding" the slack time by incorporating it into the already assigned time can help with the sense of urgency. If someone one knows that there is lax time, then lax time will be taken. However, the team and project manager are aware of this and understand the time every step should take in order to reach the deadline. 

I think it depends on what you mean by Slack Time. Unexpected delays can happen and extra time should be given for cases such as failure in Validation or Verification phases. Although it's initially costly, the time given is valuable in improving the design or for refining your test method. In the end, the product has a higher chance of being safe and effective.

If slack time means to give extra time for worker compensation, then I would use a different approach. If during the project lifetime we are ahead of schedule with extra budget, slack time could be used to give positive reinforcement to the Engineers that showed consistent effort and commitment to the project.

Slack is a very crucial element in project planning because it helps accommodate for the unexpected, like delays, vacations, holidays or various emergency closures. Including slack in your schedule also allows for increased flexibility which will prevent the project from being jeopardized when other events may occur. Minor delays also will be offset by the allocation of more resources so more slack is better when project managers are designing a timeline. Murphy's law is a very important aspect to this where if something can go wrong it will go wrong and slack is a safeguard to that 

Slack is important in case a part of a project is not going according to plan and time is needed to find the issue and deal with it. This can be the result of a variety of issues, including factors outside the project team's control such as third-party items crucial for the project being in back order. It can also include environmental issues preventing work from being done, delays between transferring things between offices, or delivery delays. Slack time also allows for better preparation between phase transitions in projects as team members will be able to have additional time to comprehend what needs to be done. In a more relevant example, cells may suddenly go bad during culturing and time is needed to investigate the issue as well as redoing the cell culture or ensuring it is fine to pass into the next phase. In another example, a batch of pharmaceuticals may be contaminated, and time is needed to redo that process. Slack should be there to be used in preparation for examples like these.