I think the same thing about that oversimplified argument as I do of the CO2 is a pollutant argument.
Overall it is clear that adding CO2 to the atmosphere, is likely to slightly help plant growth, but cause other problems and that the problems would be worse than the benefit. Still do not, however, accept the runaway, world is going to end, scare-mongering predictions of those who would pretend to want to reduce CO2, yet not even consider nuclear power.
Aslo, while you pretend as if the CO2 is plant food, is a myth and that the skeptics have been torn apart as liars, reading Dawie's article, it seems very clear that the warmers were not lying, and did base their statements on scientific studies. Furhter, given the nature of more "real-life" solutions, having less ability to control for other factors, that these results suggest less benefit, but certianly do not suggest no benefit.
In other words, you act like you have shown the warmers are nothing but liars who refuse to undderstand science, when the truth is that they based their argument on real scientific studies, that other studies sugget that they are still right, though not to the same extent.
Sort of like saying I can beat anyone in basketball, including Shaq, as long as I define beating him at basketball, as really meaning that I can beat him should he become paralyzed.
Overall you are still left with a quandry. One of the major basis for the fear of AGW has to do with our ability to feed ourselves. Now given how quickly farming equipment can be moved and farming buildings set up, the question becomes: Given all AGW predictions are true, will the overall cummulative impact of all changes with:
More farmland in high elevations and farther north
Slightly better plant growth with CO2 increase (note slightly better as per Dawei's argument as opposed to much better as per the skeptic argument.
Loss of land to desertification
Loss of land to ocean
In order to make the claims that the warmers have made in terms of food, they would have to show that the negatives far outweigh the positives, which I do not think they have convincingly done. This is all under the large assumption that their predictions are right. AS for the plant food arguemtn, I think the only reason people are getting away with calling CO2 a pollutant with the general populous, is because the general populous, do not understand the difference between CO2 and CO as Eric C stated. Further, you should be aware that the warmers are making an argument for action based upon ability to feed ourselves. The plant food argument is not an argument for inaction, but simply a retort of your reason for action based upon production of crops.
The Mr. Blob approach works if you are dealing with solutions to problem where the solution is known to be far less costly than the problem. With the solutions that have been proposed and the uncertainty in the entire CC field, this is hardly the case.