I am using Clarizen not only to "track" project progress, but also (and above all) to actually build the Schedule... That means that because my project is moving all the time, I need to be able to know what task can take longer without any problem, with small problems or with high stakes.
That should be the purpose of the Slack / float item.
I absolutely need to know what is my free float and what is my total float for my tasks, so that I know what I can move: I cannot afford to just move one task and check if something has moved...
Unfortunately, I cannot find the float / slack (whether it be free or total) attribute of a work item. I would like to suggest you adding it (I am surprised to be the only one who works that way).