AFAIK nobody has suggested that farmland could spawn non-vermin creatures. Stuff like, plump helmet men or giant moths. Unexperienced farmers wouldn't notice there's anything wrong with the plant when harvesting, then go store those spawns in inconvenient places where there may be no proper containment.
And this solves long-term difficulty how? All it does is throw super-difficult problems (solvable via military) at a new fortress that has little to no military.
The proposed solutions that actually
work have no up-front costs/difficulties and cause problems
over time if left unchecked, and is solvable by a good setup that may not be achievable from the get-go (multiple crop types + crop rotation, etc.)