There are many reason why one would want to capture the user that Actioned a task. One of them is a scenario where the Destination is a Group, now the user wants to update a list item with the user that Actioned the tasks. It would help if the designer exposed some data from the Task event like the Actioner to use in the same workflow. If you have Smartforms integrated you can do this by using the logged in user but what if the user decides to action using the worklist context menu / SmartActions instead of opening the forms.

You can get this data from Reporting Smart objects coupled with some effort. This should be something that can be done easily from a design perspective.