Insect-resistant GM corn also decreases human and animal exposure to mycotoxins, highly toxic and carcinogenic compounds made by fungi. The fungi that produce mycotoxins follow insects into plants; insect-resistant plants have no insect holes for fungi to enter and therefore no mycotoxins.
Monsanto developed GM crops that tolerate a nontoxic herbicide called glyphosate, aka Roundup. Herbicide-tolerant crops have made a major contribution to decreasing topsoil loss by facilitating no-till farming. This farming method reduces CO2 emissions from plowing and improves soil quality.
Farmers don't have to buy Monsanto seed, nor is anyone preventing them from saving and replanting any seed they want, except for patented seed they've signed an agreement not to save and plant. Farmers buy seeds from Monsanto and other ag-biotech companies because their costs decrease and their profits increase. If they didn't, farmers wouldn't buy them again.
If the popular mythology about farmer suicides, tumors and toxicity had an ounce of truth to it, these companies would long since have gone out of business. Instead, they’re taking more market share every year. There's a mismatch between mythology and reality. Maybe it's worth remembering that technology vilification is about as old as technology itself. What's new is electronic gossip and the proliferation of organizations that peddle such gossip for a living.