Unfortunately, my reply went over the Reddit limit, I'll have to continue it here:

So why do people buy them then?

Like I said before, Monsanto has cleverly managed to manipulate the market to lure in the large farmers with promises of increased yields and profitability, buy out the smaller seed producers, drive up the cost of seed, and quickly prevent anyone from choosing otherwise. They were even investigated by the US Justice deparment:

http://www.salon.com/2013/03/15/how_did_monsanto_outfox_the_obama_administration/

Highly restrictive contractual agreements that excluded rivals, alongside a multibillion-dollar spree to buy up seed companies. (http://www.huffingtonpost.com/2009/12/13/monsanto-squeezes-out-see_n_390354.html)

State officials uncovered agreements that, in one form or another, required seed breeders and retailers to favor Monsanto over its competitors. One provision, for example, prohibited seed companies from combining Monsanto’s genetic traits with the traits controlled by its rivals, unless given explicit written permission from Monsanto. Since the vast majority of U.S. corn and soybean crops contain Monsanto’s genes, the company could effectively lock out competitors.
In another arrangement Monsanto stipulated its product Roundup as the only herbicide farmers could apply to its Roundup Ready crops. Competitors say this tactic blocked a cheaper, generic herbicide from the market.(http://docs.justia.com/cases/federal/appellate-courts/cafc/04-1532/04-1532-2011-03-27.pdf) (http://dockets.justia.com/docket/texas/txwdce/5:2007cv00673/254138/)

Monsanto also promised significant rebates to seed companies that agreed to ensure its products made up at least 70 percent of certain lines of inventory. Many seed dealers have said Monsanto’s policies dissuaded them from promoting competitors’ products.(http://www.findforms.com/pdf_files/ded/35207/45-1.pdf)(http://dockets.justia.com/docket/texas/txwdce/5:2007cv00673/254138/)

Of all the antitrust efforts undertaken by the new administration, the Washington Post wrote, the Monsanto investigation appeared to “have the highest stakes, dealing as it does with the food supply and one of the nation’s largest agricultural firms.” (http://www.washingtonpost.com/wp-dyn/content/article/2009/11/28/AR2009112802471.html)

Once mimicked by its rivals, Monsanto’s strategy redrew the industry. Competition and variety have dwindled as a result. (http://farmertofarmercampaign.com/Out%20of%20Hand.FullReport.pdf)

Since the mid-1990s, the number of independent seed companies has shrunk from some 300 firms to fewer than 100.(http://www.mdpi.com/2071-1050/1/4/1266/pdf)(http://wcfcourier.com/business/local/independent-seed-companies-a-dying-breed/article_7cef1ffc-b0bb-56a8-8d83-faf894bf76ad.html)

“My big concern is that Monsanto can go out and undercut us in the marketplace through one of its own seed brands,” said the owner of a family seed business in the Midwest who asked not to be identified because he relies on Monsanto for genetic traits. “It puts us in a very vulnerable position. It could squeeze us any time.”

Despite these obvious anti-trust practises, because Monsanto is such a huge company that spends an exorbitant amount of money on lobbying, they can get away with this sort of thing:

Academics and private attorneys who consulted with the government during its investigation concede that an antitrust lawsuit against a company as strategic and politically connected as Monsanto would not have been a guaranteed win. Few firms have as methodically mastered the revolving door between Washington and industry as Monsanto – whose former employees and lobbyists frequently enjoy top posts at agencies like the Food and Drug Administration and on legislative committees – or groomed as deep ties with both Republican and Democrat administrations. The company spent close to $6 million on lobbying in 2012, more than any other agribusiness organization, and three times the sum dished out by the second-highest paying firm (http://www.opensecrets.org/news/2012/05/monsantos-deep-roots-in-washington.html) (http://books.google.com/books?id=LnOkNbVFu5kC&printsec=frontcover&dq=foodopoly&hl=en&sa=X&ei=Cn4rUZbKJYq30gHljoCgCg&ved=0CDAQ6AEwAA#v=onepage&q&f=false) (https://www.opensecrets.org/lobby/indusclient.php?id=A07&year=2012)

The moment the US Justice Department started investigating them, they started loosening some of their practises, but by then, it was too late:

Seed companies say Monsanto began loosening its licensing agreements in 2008, less than a year after the state attorneys general opened their inquiry. Months after the Justice Department followed suit in 2009, Monsanto announced it would allow farmers to continue using its leading soybeans, Roundup Ready 1, even after its patent expired in 2014. This gesture — at least in theory — opens the market to generic competition.(http://accordingtomonsanto.files.wordpress.com/2009/12/stakeholder-letter00011.pdf)(http://kunc.org/post/generic-seeds-could-have-short-lifespan)

“Monsanto had reached a place of sufficient dominance that it no longer needed its restrictive agreements, and they were just attracting trouble,” said the lawyer in the state attorney’s office. “So it loosened its practices, giving seed companies more freedom to make their own choices. But it didn’t change the direction of the market — Monsanto had already locked that in.”

But even then, you and I aren't farmers, why should we care if Monsanto has a monopoly on the seed industry? Because it increases the costs of our food:

The public will suffer the costs of Monsanto’s capture of almost total control over much of the U.S. seed business. Since 2001 the company has more than doubled the price of soybean and corn seeds, whose crops are used in foods ranging from cereal and pizza to chocolate and soda. In 2008 Monsanto officials said farmers should expect seed prices to keep rising. (http://www.nytimes.com/2010/03/12/business/12seed.html?pagewanted=all&_r=0) (http://www.dtnprogressivefarmer.com/dtnag/common/link.do;jsessionid=E2839555FFAB49946177FB788EA222F1.agfreejvm1?symbolicName=/ag/blogs/template1&blogHandle=business&blogEntryId=8a82c0bc1ae0f224011ae9296a9e005f)

“Monsanto has used its power to raise prices and retain control over genetics at the public’s expense,” says Neil Harl, agricultural economist at Iowa State University who has studied the seed industry and antitrust law for decades and consulted with the Iowa attorney general’s office during the state investigation.

These patents were supposed to encourage the production of new innovative plant varieties, but it seems to have done the exact opposite:

It is not just a matter of higher prices. The resulting loss of diversity from Monsanto’s dominance may restrict our ability to adapt plant stocks to an increasingly volatile climate. Many of the seed breeders and retailers Monsanto purchased were regional experts, familiar with the soil and adept at breeding crops suited to the vagaries of local pests and climate. That sprawling network of local knowledge and experimentation has been severely thinned. (http://farmertofarmercampaign.com/Out%20of%20Hand.FullReport.pdf) (http://www.centerforfoodsafety.org/wp-content/uploads/2013/02/Seed-Giants_final.pdf)

Kyle Stiegert, professor of agriculture and applied economics at the University of Wisconsin-Madison, says Monsanto’s degree of control forecloses important opportunities for innovation. “There are suites of traits and seed combinations that are no longer being experimented with,” he says. “We have no idea if yields could be higher if farmers had flexibility to experiment.”