Why Sound the Alarm for Resistance to Bt Corn?

Why do those against genetically modified foods (GMOs) sound the alarm whenever some research shows that a pest has developed resistance to the genetic modification?  I have often had this thought when reading about "dangers" of glyphosate-resistant weeds appearing in corn, soybean, or sorghum fields.  This same thought emerged while recently reading The Salt, NPR's food blog, in an article about the corn rootworm's resistance to "Herculex" trait in biotech (Bt) seed corn:  "Insect Experts Issue 'Urgent' Warning on Using Biotech Seeds":

For America's agricultural biotech companies, the corn rootworm is threatening to turn into their worst nightmare.

Last year, we reported that a major insect pest, the corn rootworm, had "found a chink in the armor" of genetically engineered crops. In several different places across the corn belt, the insects have developed resistance to an inserted gene that is supposed to kill them. 

Now, in a letter to the U.S. Environmental Protection Agency released this week, 22 of the nation's top experts on corn pests lay out some of the implications of this discovery, and they are potentially profound.

In order to slow down or prevent the spread of resistance, the scientists are calling for big changes in the way that biotech companies, seed dealers and farmers fight this insect. The scientists urge the agency to act "with a sense of some urgency."

(My emphasis added in bold).  The actual letter by the "insect experts" does not paint quite such a gloomy picture for Bt corn.  Instead, these scientists suggest that more acres without the anti-rootworm trait should be rotated to decrease insect resistance levels.  In addition, they suggest that some parts of the United States currently plant Bt corn with rootworm protection prophylactically, even though there is "little or no need for it."  This too, can increase rootworm resistance levels.  The entire letter is short, very interesting. and can be found here:  Memorandum to Open Docket Plant-Incorporated Protectant Insect Resistance Management (IRM),

This brings me back to my point.  Unless you are the farmer paying for expensive seed corn that may not be  effective against the corn rootworm, why is pest-resistance cause for concern?  Corn without insect protection is nothing new.