Eric - it has to do with opportunity costs... spending money on one thing at the expense of a more useful other thing. If we (humanity) spend a few trillion dollars to reduce CO2 a few parts per million and it turns out to have a null effect on global climates, we haven't used that few trillion to do things like improve water sources, advance medical discoveries, reduce famine, and so on.
Worse yet, we could pursue routes that might actually be detrimental in the long run. For example, you've likely heard about the evils of logging sometime in your lifetime -- did you know there are more forests now than 100 years ago? The commercial pursuits of the logging industry has *added* to forestation, contrary to old time protestor beliefs.