You can learn from anyone, if you have the attitude and desire to learn, if you can keep "beginners mind". However learning from a fool about what not to do was not what the fool was trying to teach. In fact he or she was most likely not intending to teach anything. Before discussing the ethics of teaching should we not first ascertain if the individual has intellectual and technical competence married with good pedagogics. I have trained with some shihans who were technically brilliant, but were very poor teachers, unable to bring across what we needed to learn in order to do what we are being shown. I have practised martial arts for 30 years and have taught aikido for the last 9 and I begin to understand just how difficult it is to really "teach" anything. I tend to believe increasingly in the old fashioned view that people must "steal" the art, which requires a special blend of dedicated "learners" and "teachers" in the right place at the right time. I personally distrust the overly liberal idea that everyone is an instructor, too many people just can't shut up and train!
So "good" students are actually "teaching" themselves, no?