On average most corn fields will benefit economically from a trangenic corn hybrid. Thus, hybrid trials are still as important as ever. A major concern with these transgenic crops is the potential for development of resistance of corn borer to the Bt toxin if they are over used. Most insect populations, if they are exposed to a toxin excessively, can develop resistance, making pest control products less effective. Any potato grower knows of this problem with Colorado potato beetle.
Resistance is also common in many other crops, including cotton and apples.
Fortunately, however, corn borer has not shown resistance to any chemicals to date. Specifically, seed companies will probably be specifically recommending that you not plant all your acres to transgenics. Planting too many acres to transgenic corn may lead to resistance.