Because any such exaggeration, whether witting or unwitting, will be used by the alarmists and their allies in the media as a tool to invalidate all criticisms of so-called "catastrophic" climate change.
And as the graph below makes clear, you don't have to exaggerate. Real world observations are still at variance to the projections of the IPCC's models. Actual recorded global temperatures are even below the projected ones assuming no growth in atmospheric carbon dioxide at all, (whereas levels have continued to increase without let up).
Mugged by cooling temperatures
Lucia’s Blackboard says the cooling climate does contradict the predictions of the IPCC alarmists, but not by as much as Christopher Monckton claims.
Hers is a needed reminder that in standing against serial exaggerators it is important not to exaggerate oneself. But this is also a good excuse to plug Lucia’s terrific line of presents for sceptics, like her famous mug:
Warming predictions are in red, and real world temperatures in blue and green. Hit the link for other gift ideas.