Note that the primary purpose of the algorithm that assigns a default value to a work item in a plan view is to pick a value that will leave that work item in the plan within which you created it.   Otherwise, when you refresh the plan view, the work item you created would disappear from the view.  

Geoff, you can use subcategories and even group by them in the plan. We use this e.g. to use the filed against in the RTC development and to provide a lot more detailed information about the problem area.

In the hands of RTC experts, I agree that sub-categories can be added without harming the primary purpose of automatically mapping a work item to the responsible team area.  But in my experience, unless your primary focus is defining categories which are understandable by the work item creators and which automatically map to the responsible team, you end up with a poor set of categories.

Thanks Geoff.
The use case is that the user maps multiple sub categories to one team area, and they notice whenever they create the workitem within a plan view(scope to that mapped team area), it will always go to the last category(this is in v6.0.2 they use). I see similar behavior here. They want to know if there is any way they can actually set a default category so that they can be used by default for the work items created in plan view in this case. As per the info from Ralph, this does not seem to be possible.