How often do you promise something to others that you don’t intend to do anyway? Maybe a salesperson calls you and you don’t want to be rude. Maybe you don’t want to go out on a date but don’t want to appear insensitive. Maybe a client has asked you for a favour and you’re afraid to lose the client if you don’t agree to it. So you say “yes” but hope that the other person will forget to call you on your promise.
Your “politeness” is wasting your time and theirs. It also kills trust. Say a clear “yes” if you really mean it and then do as you said. Otherwise, realize that if you give a clear, yet diplomatic “no, thank you for asking but I cannot do this”, you let them know where they stand.
If you don’t intend to follow through anyway, it is better if they know now so that they can make alternate arrangements sooner!