My goodness there is a lot of attention paid to motives in this debate. But why? It seems to me that judging from the emotional intensity of many on both sides, motives are absolutely rampant. So? Who among us is not motivated by something to be involved in this debate. Some may be motivated by scientific and intellectual interests, some by religious convictions, others to defend a worldview, and still others for the thrill of battle. While some motives might be considered more noble than others, who would even want to debate someone with no strong motive to engage.
Seems to me that resorting to an argument-from-motive at best reflects a laziness to address the issues, and at worst is an unscrupulous attempt to cast aspersions on the intellectual integrity of someone. Or even worse yet an appeal that whole groups who hold certain worldviews are ignorant, stupid, or incorrigible somehow.
Fact of the matter is there are no fields of exploration where motives have no play. Certainly personal motives create a bias, but that in and of itself is not a negative. Instead it provides the impetus and drive for those explorations. Can motives result in distortions? Sure. However, I think history has shown that those "who get it right" are somehow able to hold their subjective bias in abeyance enough to "let the argument and data talk".
An argument-from-motive is like a racist remark that appeals to a stereotype to sway opinion, but is really an attempt to avoid engaging in debate with integrity. It's an easy way out but when it becomes a mantra, as it has, it is also so transparent to weakness that it loses points instead of gaining them. In the final analysis it will be the power of arguments that win the day. Appeals to motives will only delay the inevitable.