Summary of the above video for those short on time: The police never say "Anything you say will be used to defend you in court" because the phrase is "used against you" - it is against - always.
Anything you said that might be helpful to you can be left out - they have no duty to disclose anything that counts in your favour, and any tiny slip-up you make may be quoted out-of-context.
Worse still, if it is just "a casual chat", then it means that there is no formal record of what you said. They can claim you said absolutely anything in order for them to "tidy-up" their case against you.
The video has some shocking examples and is very thought-provoking.