Richard Betts posted some further thoughts on GCMs and public policy in the previous post on this subject. Since the thread is now heading for 300 comments I thought I'd post his ideas up here and respond in turn.
Richard first set out his understanding of my position.
I'd initially thought that you were claiming that the very need for any kind of climate policy was based on GCMs. Clearly it isn't, for the reasons I stated, but it seems this isn't your point here anyway. You seem to be moving a step further and talking about the importance of GCMs to the details of climate policy (eg. a carbon tax). Here I do partially agree with you - GCMs do of course play a role in the details, as they help with understanding the climate system, but they are by no means the only source of information. Moreover, I don't think the examples you give would be substantially affected if we didn't have GCMs.