The style of argument you call "Ad Hominem" should really be called "Attacking the author's credibility". The original meaning of ad hominem was not attaching the author but rather appealing to the readers emotions rather than logic. Hence, "think of all the children who will starve due to biofuels", which contains the embedded assumption that biofuels will cause starvation but does not support that argument. In most cases attacks on the author's credibility are useful and valid, since the reader will tend to rely on evidence as presented. In addition, the motivation of the author is relevant since most people decide their position on an issue and then seek supporting evidence rather than surveying all of the evidence before making up their minds.
Attacking someone's credibility has no bearing on that person's argument. A true statement spoken by the devil doesn't make that statement false.
Next, it's very hard for someone to correctly divine the motives of another. It is often the case that it's easier to make up base motives and assign then to an opponent than it is to refute their argument.
Finally, that people "decide their position on an issue and then seek supporting evidence" simply shows that "people" are idiots.
