This makes a society weaker - it's effectively a monoculture that restricts the data available to the decision makers.
It also reduces a societies ability to innovate in art, culture and eventually technology and science - innovation requires the ability to take an outside view, to be self-critical, and a society that is too busy forging a 'strong collective identity' will make it harder for individuals to do that.
It even poisons a societies own understanding of itself and its history, since we have so far failed to come up with good ways of building a 'strong collective identity' without accepting and promoting lies about history.