Why make tasks visibile in the portal?
Well, often we ask customers to do something, like check the that a custom report is correct. What we did to get around this problem was to set the workflow step of a feature or incident to 'Client' then they know that they have the responsibility for the next action.
Where tasks would be helpful is in the project planning stage, for example, we know that when we implement a new system, the customer has to do a whole series of tasks, if we could make them visible on the portal then we could create tasks which could then be assiged to them.