The % complete progress bar in the Tasks widget is misleading, at best.
It doesn't account for tasks that may be added by workflow after its current state (it can't really), so it ends up jumping around between completion percentages (0 - 100 - 50% for example). This gives a misleading sense of how far along the incident is in completion, and to all intents and purposes it's useless (sorry).
I'm not sure it's fixable in its current form (it can't possibly know how many tasks will be in a workflow-driven incident playbook) so it needs to do something else or go away completely to avoid the confusion.
In the case of wrong results, I wonder could these be caused by inactive tasks being counted (tasks added by rules, then made inactive when the rule re-evaluates)...if so thats a defect in my view and should be raised as such.
As to the original request, as you say, we can't predict which way a playbook will evolve, so the task completion can only be accurate in the context of the current state of the playbook. We would not propose removing it as thats still a relevant view at that moment in time.
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I forgot - sometimes it's plain wrong. I've seen cases where 3 out of 6 tasks are closed and it claims completion is 67%.
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