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Well, I guess this would mean that they don't take up dex space.
Although, I would prefer to see it as a form taken as the pokemon is sent out, instead of the change happening during the battle. If that's the case, it would be like Zen Mode, and Zen Mode is awful.
Also, it would make these forms easier to deal with competitively. I don't want to bring an Ampharos or Mawile into battle, let it lose half of it's HP, and only then be useful. Those pokemon have enough trouble staying alive already.