And they could just as easily have cutting pliers with them for your cable, or bolt cutters for your padlock.
They could, although an allen key that would detach a seatpost in order to steal it takes up a lot less space than a heavy duty bolt cutter. Someone fiddling with a seatpost doesn't arouse suspicion in the same way that someone wielding a bolt cutter would.
I agree we all have to figure our own "good enough" point although, given the relative ease of putting a couple of allen keys in your pocket or carrying around an angle grinder, my "good enough" point would require more than an allen key to thwart.