Should an interface ever say “please”?
Avoid the word "please," except in situations in which the user is asked to do something inconvenient (such as waiting) or the software is to blame for the situation.
Correct: Please wait while Windows copies the files to your computer.
I always use "please" in my error message for it to be polite, like
... please enter a value in the username field.
Why do we need to avoid the word "please" for errors like this? Won't adding "please" make the message more polite?