Halliday & Matthiessen (1999: 558-9):
But at the opposite corner, so to speak, if we combine low value with oblique (remote), the result is typically blending rather than ambiguity: e.g. it couldn't hurt you to apologise is a blend of 'it would not be able to hurt you' (readiness: ability), 'it is unlikely that it would hurt you' (probability) and even perhaps 'it would not be allowed to hurt you' (obligation). In other words, looking at it from the point of view of blending, in the region of 'what I think’/ 'what is wanted', it is easiest to blend the low values 'what I can conceive of’ with 'what is permitted', especially in 'remote' conditions (hypothetical, projected or tentative) [realised as could, might]; and hardest to blend the high values 'what I am convinced of’ with 'what is required', especially when 'immediate' [realised as must].