I'm not impressed with her recipes bar the Victoria sandwich one. I tried her chocolate cupcake recipe a couple of times, where she asks you to mix cocoa powder with hot water and add it to the egg, burtter and sugar mixture. It always, always went wrong. The cocoa powder wouldn't dissolve properly and end up lumpy, making the cupcakes rather heavy and full of bitter cocoa lumps.
Anyway, long story short, I think MB is an average cook at best. If you carefully listen to her comments throughout Bakeoff, you'll see that anyone can come up with them, there is no 'expertise' to speak of.
I agree with a pp that he 'value' if any, is in her being a wholesome role model, not a great cook.