I don't really think that the idea that something must be perfect to be useful is correct. Trying to find a perfectly accurate model is impossible, so you must qualify your meaning of the phrase. Mathematics has a whole subject named statistics which spends quite a lot of its time dealing with the proportional incorrectness of models. If you look at the graphs or figures in the report (not just the headline of +4 degrees celcius), you will notice that almost all of them have a degree of uncertainty; figures without uncertainty values are as good as useless.

Also, if you are going to use "No one challenged me and I got several upvotes and follow-on posts in agreement" as a pseudo appeal to authority, I don't think that the readership of a Hacker News comments section really qualifies as authority on this particular issue.

Edit: Oh, apparently models have already predicted temperatures since 1900 correctly via hindcasting, along with changes in climates after significant events such as the eruption of Mount Pinatubo. And so far, they seem to be on-track with empirical data: http://www.skepticalscience.com/images/SLR_models_obs.gif, though I fully understand if you want to discard that graph due to it's source; the risk of selection bias is very high when taking a shallow look at a large and deep subject.