We need diversity in our plants to be safe from massive amounts of any harvest being wiped out by a single virus/adverse weather condition.
The more uniform a crop is, the more vulnerable we are to some kind of blight taking out a significant chunk of our global food supply.
We do have seed banks, but living populations are better; over time fewer and fewer kept seeds will grow into healthy plants.
Genetic diversity of our plants is so crucial to our survival as a species it cannot be dictated to us by greedy mega-corps who make genetic alterations and then treat it like any other patentable goods.
They make "terminator crops" whose seeds cannot be used, yet pollen travels on the wind.
There are good uses for genetic engineering, but we need to work with mother nature, enhancing genetic diversity, keeping our heritage cultivars alive for future generations.