I suspect that the reason Mack Boring advises against putting the engine in reverse is the possibility of damage when you try to shift back to neutral.

As I said earlier the water pressure on the prop creates a torque that makes it difficult to shift out of reverse into neutral. The higher the boat speed under sail the greater the torque. If you just start in gear (which reverses the torque as the prop shaft starts to turn) or head up into the wind to reduce speed then you can easily shift into neutral.

If you are aware of this possibility and avoid it, I don't think that there is any harm in putting the transmission in reverse while sailing.

David