Many agrarian cultures have a history of seed sharing and seed saving, which Monsanto seed (and lawyers) prevents. Over time, the poor nation of Elbonia's farmers cannot plant anything _but_ Monsanto seeds (as they have no uncontaminated seed banks, and their Monsanto crops are sterile). At that point, the country is entirely dependent on a multinational corporation in order to grow food. They have no seeds (genetic resources for plants) with which to plant new crops, to breed with other plants to make disease or drought-resistant hybrids, and in general a collection of diverse breeds of that particular plant are in time replaced with a single breed.
Some would argue that this sort of behavior is unethical and abhorrent. There are frequent stories from farmers of being bullied by Monsanto, which makes this an even easier opinion to hold.
Others point out that without GMO seeds, it would be very hard for some of these countries to plant __enough__ crops to feed themselves, regardless of diversity. From a standpoint of feeding a planet, that argument seems to carry some weight as well. It's a complex issue.