i encountered this sort of "pay with a tweet" (pwat?) via a macheist promo you're mentioning and i had exactly the same reaction as actually paying cash, except that it was more difficult because it was a sort of half-hearted, monetarily encouraged plug for something i wasn't internally raving about. At the point i actually did it, i felt like i had sold out.

Which is to say that pwat could be totally effective in being a payment mechanism, but only if the stakes of the pwat's message are high enough to equate to a non-trivial reblogging of a url.