Most farmers already buy their seeds from somewhere anyway even if their non GMO
Sometimes, that doesn't matter much - I've saved corn seed and replanted it year after year, and it may not be exactly Silver Queen or Seneca Horizon anymore, but it's still corn. But some things get weird.
Also, a number of these cultivars can cross-pollinate, and then you can get some really interesting monstrosities. For instance pumpkins getting crossed up with buttercup squash that come out as dark green and orange tiger-striped things.