Yes, it could be the case that the best pure benefit causes have less positive impact than the best mixed benefit causes. But I have no special reason to believe this is the case. If benefit of the doubt is going to go to one side without argument, I would put it on the side of pure benefit causes, since they don't have the additional negative factor.

By taking disagreements seriously, I mean that I think that if someone disagrees with me, there is a good chance that there is something right about what he is saying, and especially in issues of policy (i.e. I don't expect people to advocate policies that are 100% bad, with extremely rare exceptions.)

That global warming is happening, and that human beings are a substantial part of the cause, is certainly true. This isn't an issue of policy in itself, and I don't take disagreement about it very seriously in comparison to most disagreements. However, there still may be some truth in the position of people who disagree, e.g. there is a good chance that the effects will end up being not as bad as generally predicted. A broad outside view also suggests this, as for example in previous environmental issues such as the Kuwait oil fires, or the Y2K computer issue, and so on.

In any case the kind of disagreement I was talking about was about policy, and as I said I don't generally expect people other than Hitler to advocate purely evil policies. Restricting carbon emissions, for example, may be a benefit overall, but it has harmful effects as well, and that is precisely the reason why some people would oppose it.