I'll even go a step further and say that not only do you need slack, you need to scale up your slack as the slack gets used. It's dangerous to treat your slack like it's spare capacity for a rainy day: more often than not, when you dip into your slack to handle interruptions and unexpected work, much of that slack doesn't return to being slack when the interruption is over. Often that means more hiring, which means budgeting. If you didn't plan to scale up your slack, you probably don't have budget for it, which means either making hard choices or running low on slack (leading to a feedback loop that eventually ends in collapse).
I'll even go a step further and say that not only do you need slack, you need to scale up your slack as the slack gets used. It's dangerous to treat your slack like it's spare capacity for a rainy day: more often than not, when you dip into your slack to handle interruptions and unexpected work, much of that slack doesn't return to being slack when the interruption is over. Often that means more hiring, which means budgeting. If you didn't plan to scale up your slack, you probably don't have budget for it, which means either making hard choices or running low on slack (leading to a feedback loop that eventually ends in collapse).