Unless someone can tell me otherwise, he's my understanding.......
As parts get used, they become "happy" with accepting loads in a certain direction (gear teeth for example); sudden and violent loads in the opposite direction do them no good at all and you are more likely to get a part breaking.
It's not just the transmission that tries to reverse itself, crank, pistons, cam, rockers - etc. etc.
I view it as putting loads into bits that would be happier not having to do that work.
BTW - I drive cars with more delicate transmissions than a tractor
I raced 2-strokes for years, so I admit I'm possibly a bit OTT in that department. A pre-mix MXer might "nip-up" a bit if you used the engine as a brake.....