David Weinberger proposes a distinction between loud rhetoric and propaganda: Propaganda is wrong and not based on facts whereas loud rhetoric is ready to fall back to not so loud rhetoric. I see the point, but my feeling is that the 'not so loud rhetoric' that one can fall back on has about the same effect as the fine print below the beautiful images of happy children and the amazing speed reading after the product praise that happens at the end of pharmaceutical ads: Nobody reads it or listens to it. It's just there, so that when somebody complains you can fall back to that text and say 'I told you so'.
I admire the freshness and aggresiveness of loud expression too, (e.g. someone like Barbara Kruger), but I think it has a certain poisoning effect also that is particularly dangerous in the current political climate.

Oh, and as to Martin Luther King: I think 'I have a dream' is about as far from 'slick' as you can get. Powerful rhetoric, sure - but clearly not at all in the same vein as the kind of attack rhetoric I'm criticizing.