Just because we didn't take a Lb or Cb doesn't mean we cannot get a decent one in free agency or via trades. Mayhew and Schwartz have a plan and they are sticking to the plan.
As for LB, there were rumors that Stephen Nicolas was interested in Lions (according to rotoworld.com), Plus Stephen Tulloch from Titans could be a free agent or could be available for a trade. There were rumors of Lofa Tatupu being on the outs with the Seahawks. In the end there are still ways we could go and get decent linebackers that will be more productive than some of the LBer's that would need developing.
Corner- I think the Fairley pick was smart. How many Cb's want to have an elite defense line and such. If free agency happens we could see some small moves that will create value. I know Schwartz/ Mayhew say that they won't make big splashes in free agency, but even the smallest stone can make some big ripples. Also, Schwartz and Mayhew know how to pick cb's, they may be able to get a corner late in the draft that will be the next Cortland Finnegan. It could happen.