Task Dialog package actions control the UI available for each type of task. Different kinds of tasks may require different kinds of actions to be applied to content and the package as a whole.
Permissions are also adhered to - so, if an update action is available in the task, but the user does not have update rights, they can't update.
I think it's better to be explicit about the list of actions that can be applied in a particular workflow context, rather than imply them from permissions. The rest of the Web Client behaves this way.