2. Not all plants are patent-able, only ones that reproduce asexually. Monansto/DuPont don't patent varieties of corn or soybeans as they don't reproduce asexually, but they DO patent gene insertions (aka BioTech Traits, or BT traits). Getting a gene that kills rootworm or has glyphosate (roundup) resistance is mind bogglingly expensive. When a variety has multiple traits inserted you'll hear it called "a stack". These high cost, high benefit efforts are precisely the kinds of things we would want to be patent-able to encourage companies to invest the huge amount of resources.
The crop yield may be lower, but the cost of inputs (e.g. chemicals) will also be lower. If the goal is to maximize farmer profit, then there may be a more optimal solution that uses greater genetic diversity.
Usually when you hear about people worry about "the mono-culture" they are advocating that we plant more of a different types of plants (canola, sunflower, sorghum, wheat), not different varieties of corn/beans.
Also, that they are all identical is bit of a misnomer. They are, but they are exceptionally strong, vigorous plants (google "hybrid vigor" or "heterosis").
Update: Go ahead, save seeds from your hybrid tomatoes!
http://blog.arrowheadalpines.com/2011/08/go-ahead-save-seeds...