The problem with manners is that it is always the case that the people who perpetuate them create arbitrary links between manners and a positive characteristic, and then demonize those who do not subscribe to the manner as people who do not possess the positive characteristic, even though there is no direct logical implication.
For example, people who care about manners will allege that eating with your hands is barbaric, when in fact eating with your hands is not barbaric as long as you clean your hands afterwards. But due the the arbitrary link perpetuated by manners, certain cultures were unfairly viewed as barbaric.
Manners perpetuates the idea that it is okay to infer characteristics of people based on arbitrary predefined implications of their behavior, it is illogical and a form of prejudice created to oppress people of different class and cultures. And even when manners gets it right, it is still wrong because of the reasoning behind it.
For example, it is good manners to respect the waiting line and take your place at the end of it. But not cutting the line isn’t good “just because”, it is good because order is good for efficiency and fairness, and after you know that whether or not its a good manner is entirely irrelevant. And if, in the future some technological change or better way of managing a queue comes along, waiting in a line would cease to be a good idea and people should stop doing it.
And that’s another problem, manners are in effect part of culture and belief. And the problem with culture and belief is that people who like culture for culture are unable to let go when the principles behind their culture are disproven and obsolete. While a rain dance is interesting to watch every now and then, whatever importance rain dances had was lost as soon as people realized there is no causal relation between dancing and the weather.