If we don't eat them they will not survive
Promoting the eating of rare breeds pork may, at first, seem an odd way of conserving them. But unlike wild animals, farm stock has to pay its way.
In fact the only way these breeds will survive is if we eat their non-breeding produce. These are farm animals as opposed to tigers or pandas and need a market to encourage people to keep them. The more demand there is for their produce, the less rare they become. In other words, by eating them you are helping to save them preserving our heritage.