Modern people spend a lot of time at watching commercial TV. Watching commercial TV is a large part of their entertainment activities for modern people. As a result, a lot of modern people cannot live without commercial TV. Some people have learned some knowledge from commercial TV, and even have it as the only source of knowledge. However, I think commercial TV is bad for the brain if people rely heavily on it for three reasons.
First of all, relying heavily on commercial TV makes people tend to learn things too superficially. A lot of people expand their knowledge from watching commercial TV, especially knowledgeable TV channels, which make knowledge easier to understand for most people. But those channels may sacrifice depth of content in their programs. If people rely heavily on them for learning, they may have less chance to learn a subject deeply. People need other sources such as books and journals to learn further about a subject, which is most likely not provided by commercial TV.
Second, relying heavily on commercial TV makes people tend to learn things slowly. Commercial TV has good visual and aural effects to interest people, and to make things more understandable. Nonetheless, commercial TV programs are not self-paced, thus they do not perfectly match the learning ability of everyone. People who already have some background knowledge about a certain subject would most likely think the pace of those programs too slow. Though those programs can be recorded and repeated, they are still inefficient, requiring a lot of time to reach the useful places. Therefore, if people rely on commercial TV for news and knowledge, it is too inefficient, and makes the brain ashamed since the brain can do much better than just waiting to be fed by a rectangular screen.
Third, spending too much time watching commercial TV for entertainment makes the brain degenerate. The brain needs stimulation, something hard and complex, to train it and to bring its talent into full play. The entertainments provided by commercial TV probably make people happy, but definitely make the brain sorry. If people spend too much time enjoying the silly fun of commercial TV, they will possibly become more and more stupid.
In brief, though commercial TV provides knowledge and entertainment, relying heavily on it may make people tend to learn things superficially, dramatically slow down the speed of learning, and make people become stupid little by little. Therefore, commercial TV is bad for the brain if people heavily rely on it.