Mary, Queen of Scots, said that a weak encryption can be worse than no encryption at all. Mary and Babington started with a good encryption but as cryptanalysis progressed in England they failed to change there code and make it stronger. This allowed Queen Elizabeth's men to crack the encryption and forge letters to Mary and Babington.
This ability to crack the code and use it against your enemy is what Mary was warning of. Their weakened code was cracked with ease and Mary and Babington were unaware that their cypher had been broken. Mary and Babington were placed in a false sense of security that left them writing the entirety of the plans in their letters. When Walsingham's men discovered the letters and broke the encryption, they knew they had sufficient evidence to execute Mary and Babington.
Mary and Babington were not prepared for the encryption to be broken. They believed that hiding the letters and encrypting them were sufficient when in reality they had led themselves to their own demise. A weak encryption is far more worse than no encryption at all because you place yourself in a false sense of security and rely heavily on the strength of your encryption to keep you safe. As seen in the case of Queen Mary, she was killed because she did not prepare for the worst.