One may assume that any type of encryption is better than no encryption, but for many situations, that may not be the case. Take the story of Mary Queen of Scots. Her weakly encrypted correspondence with Babington was interpreted by expert cryptanalysis Phelippes, leading to her eventual execution. Mary and Babington were so confident in their substitution cypher that they explicitly spoke about their plans to assassinate Queen Elizabeth. Unbeknownst to them, there had been huge advances in the field of cypher-breaking. Had Mary and Babington possessed an accurate sense in the weakness of their cypher, their conversation would have been far more discreet, taking care to discuss their plans in a more cautious manner.

This form of explicit versus discreet communication can be seen in everyday situations where cyphers are not involved. For example, if a group of bilingual people want to talk about someone nearby without their knowledge, the group will most likely switch to the second, less widely-spoken language. They would talk about said person without any filters, as they'd assume that no one around them would be able to understand what they are saying. However, if the group of people aren't lucky enough to have a second language to fall back on, they will probably communicate in a more discreet manner, with facial expressions and gestures, rather than clearly spoken words.

The case of Mary Queen of Scots could be a lesson to anyone who wishes to communicate through encryption- communicate as if your cypher is breakable, no matter how secure you might think it is.