I often need some way of selecting some but not all tasks in a single action list as well. This could be done as suggested, or (gasp) allowing priorities.
The reason is straightforward - many of my single action lists have 10 to 20 items on them. For instance my "admin" tasks (check email, sort papers, return headset, return switch, unsubscribe to google checkout, submit timesheet...) right now sits at 20 actions.
I have always prioritized my task lists into four buckets - must do today; do today if time is available; not for today; not for this week. The latter two could collapse into an "on hold" classification, but I don't find it easy to filter down to the 3 admin actions I must take today; the 3 I'll do if time's available, and not see the remaining 14. What I've done is to create two single action lists - admin today and admin not today, and set the review to daily - but this is sort of a kludge.
(And unfortunately between my personal, church, and work top-level folders (or areas of my life) there are probably 6 different single action lists that are like this.)
So that's sort of a "use case" for some way to classify actions.
BTW, as another use case, at the end of the review process, I have perhaps 30 work and 30 personal projects; some of which are parallel and some sequential. Looking at remaining actions gives me somewhat of an overwhelming list. My usual mode of working is to print out the silly thing and mark with a red pen the ones I'm going to work on first. Simple priorities would help me select between the actions available to me; I can't simply use "next action" because I have a number of parallel projects, and next action simply picks the task that happens to be on top.