If "soft power" is, as I understand it (perhaps mistakenly) essentially an entity's power to attract other entities for what it is, then why does such an entity (call it a nation like the USA) require "public diplomacy" to attract other nations? If we Americans seduce others because of who we are, then why need we "sell" ourselves to others through PD? Indeed, countries that use public diplomacy, it could be argued, have a deficit of "attractive power" and thus feel compelled to make up for it through government-supported programs that present and represent them overseas "positively." Take the USSR, with its vulgar propaganda, during the Cold War -- or China seeking to display itself favorably to Americans with a promotional video on six huge screens in Times Square.
Is not, conceivably, "soft power" an argument against the very notion of "public diplomacy"?
The below reflects my speculations better than I ever could:
"Soft power is, after all, like sex appeal on a national scale: it is more a reflection of who you are than how you talk about yourself, and if you say you have it, you probably don’t."
--Blogger David Wolf; image from