If "politeness" dilutes the message, then to hell with it. If
you can be polite /and/ get your message across, so much the better.
But the message comes first.
Here's a practical example of what I mean: someone recently
approached me selling candy bars to support a Boy Scout troop. A
polite "no thank you" would probably have been sufficient to spare
his feelings and my $1, but it would not have educated either of
us about our respective values. In fact, since his question was
"Would you like to buy a candy bar for $1", that simple reply would
have been a lie, because I would indeed have liked to buy it if it
benefitted, say, a private school. I said "I'm sorry, but I don't
believe in funding an organization that would not allow me to join."
That might make him uncomfortable, but he gains a direct experience
of his choice to support such a discriminatory organization, and I
had the opportunity to hear his thoughts on the subject if he chose
to respond (in this case he didn't--he simply said "OK" and went on).