Just like we can cancel a render - we should be able to abort any command. Occasionally you accidently or on purpose start something that is way too complicated - and it would be good to be able to cancel out of it - essentially a preemptive undo.
This will become possible once geometry and mathematics calculations are converted to multicore. For now, it is not possible since the same core that is doing the thing it was asked to do is the same one that monitors input of a STOP command. Luckily however, this is effectively a guaranteed feature though once core support comes.