In another thread, a commenter asked an interesting question: what is the Earth's ideal temperature? This question is often used as a version of the "AGW might be good argument": the point was that perhaps the ideal temperature was warmer than today's, so climate change might actually be beneficial.
Given everything else equal, one might be able to make an argument that a warmer world might be better. However, everything else is NOT equal. Rather, we have adapted to our present climate and made significant investments in these adaptations. We build cities in places where it makes sense to put them (with the notable exception of New Orleans), we build infrastructure where it makes sense to put infrastructure, we perform economic activity where such activity makes sense. If the climate changes significantly, it might take enormous investments to adapt to our new climate. Our previous investments in infrastructure might become worthless.
Consider this example. You build a sawmill next to a roaring river, using the river as a power source for the mill. Now AGW causes the precipitation pattern to change and the river moves miles away. In this new world, the river might have higher flow rates, which would provide MORE energy to the mill, which would be a good thing --- if the mill were still on the river. But the mill is no longer anywhere near the river. So you have to build a new mill on the banks of the new river. This investment in new infrastructure is potentially expensive, and some countries will not be able to afford it.
My point here is that any question about suitability of future climates has to take into account our investment and adaptation to our present climate.