Re: politeness or no..
Wrap your message in language that will allow it to be delivered and properly received by the other party. That's kinda what communication is--sending and receiving messages.
If it takes politeness and sugarcoating, go for it. If it takes a blunt delivery, go for it. There's no one-size-fits-all to communication.
I treat that as an ideal, of course. There's no hope of reaching some people if you have to bend your delivery too far to cater to them.
An example where I wish people would just be freakin' direct and blunt and get to the damn point already:
"Well.. we had a lot of great contestants today.. and it was hard to choose a winner because everyone performed so well.. and the judges were afraid of having made the wrong choice and they feel bad for those who didn't win.. and [10 minutes of bullshit].. and the winner is [some guy] but the rest of you did well I swear to God."