They become not so good when they become our primary input. When we ask the question what does my favorite author say about… or what does my favorite speaker say about… We need to be engaged personally with the Word. Anything our “favorites” say is filtered through their experiences and gifts. While it may be really good, spending time studying our “favorites” is a pale reflection of our spending time with the Holy Spirit in the Word that He inspired.
It would be like settling for a picture of Everest when we could go experience the mountain ourselves.