In defense of spite
As an ethical plant eater (i.e. I think there's a moral reason incumbent on everyone to reduce the amount of animal-eating, as opposed to simply having a personal dislike for animal-eating), I certainly would prefer people not eat additional animals to spite PeTA. But the comeback from the vegan side strikes me as question-begging. It presupposes that eating animals is prima facie bad, and hence something that would require a stronger argument than spite to justify. But Barnett et al. don't share that view.
Imagine if these ads had been put out by People for Encouraging Temperance in America, and urged pro-lifers and pro-choicers both to give up alcohol. I think it would be perfectly reasonable to open up the liquor cabinet and do a shot in their "honor," to spite them for the offensive way they tried to push their anti-alcohol message. Because while I don't drink alcohol, my teetotalling is a personal preference, not a moral duty, and so I don't think responsible alcohol consumption needs any stronger of a reason than spite. In the temperance hypothetical, the spite is purely for the amusement of the spite-er. But it's only a short step from this kind of spite to tactics that really do put a dent in the spite-ee's cause, such as promises of the form "for every anti-gay protester that shows up, I'll donate $10 to GLAAD!" which are widely (and rightly) approved among progressive bloggers.
In this connection, it's interesting to note moderator SKM's explanation when she asked Barnett et al. to cool it on the spite comments -- she argued not that eating animals out of spite was bad, but that it was "squicking out the vegans," i.e. bothering people who ought to be tolerated but who are not providing compelling reasons to share the perspective from which foie gras eating is squicky.
So while I hope nobody who's (rightly) offended by PeTA's ads goes out and eats foie gras out of spite, I don't think that's any better or worse a reason than if they ate the same foie gras because it's tasty.