We've covered the issue of how nature is finding a way around the engineered intended benefits of GM crops, with pests developing resistance to them, many times. Here's another straw on the camel's back, via Mother Jones:
New research in Environment Science Europe shows that rather than Roundup Ready crops decreasing herbicide use, as intended and proudly touted by Monsanto, since 1996 when these crops were first commercially used herbicide use as gone up 11%—with yearly increases markedly increasing in the past several years, after some initial reductions.
From 1996 to 1999, Roundup Ready crops did result in a 2% decline in herbicide, but since then as resistance has developed, the opposite has been the case. In 2002, for instance, herbicide use for these GM soybeans was up 21%. In 2009-2010, it was up 24%.
In terms of insecticides, the research found that seeds designed to be toxic to insects, Bt seeds, have reduced use in insecticides somewhat, but not anywhere near to offset the increases in herbicides.
Taken together, from 1996-2011, pesticide use in US fields has climbed 7%, or an additional 404 million pounds applied.
In the Mother Jones piece Tom Philpott goes into much greater details about the research and what it's leading farmers to do in response (sometimes using older, more toxic herbicides)—so read that, or the original research, if you're so inclined.
For the average green watcher though, the takeaway is: Since their introduction, genetically engineered Roundup Ready crops have not delivered the herbicide reduction benefits as intended, with the trend in herbicide use going up, as resistance develops.