I believe a radial action menu would be useful during an operation that has more than one possible outcome that is on something already in the game that's dynamic, I.e: opening a door or entering a vehicle. This action menu could be accessed when a key is held down only, vice just clicked to activate the object.
A good example would be entering the driver position of a vehicle. You could press spacebar to just enter it (into the drivers seat with the icon shown), or you could hold down spacebar and get a radial menu to enter other positions available.
A door could be opened immediately, or allowed to crack open, place a charge on the door, ect.
In these contexts this would be a perfect solution.
Now if you didn't want to select something and needed to move you mouse you would just let go of spacebar or whatever key is assigned to the action button. If you are being shot at while selecting a vehicle position a radial menu would be less clumsy then the current setup is now, as you could just mouse to what you wanted to do vice scroll through a possibly ever changing list, or just release spacebar and fire back without needing to hit escape.
For those with the argument that it ties up your mouse & movement, I would like to see a scenario when you need to move your mouse around or yourself while entering a vehicle. Currently, its only needed because you are required to dance around a vehicle to trigger the correct options for the current system's menu...