The only logical way to make sense of the reluctance of many vegetarians to back IVM is that their choices are not as driven by animal welfare and environmental considerations as we — and they — assume. Perhaps a distate for eating meat is a visceral feeling that is only loosely connected to a ethically motivated imperative not to cause undue suffering to animals. Many people cannot distinguish between their ‘all-things-considered’ moral judgment and their unmediated gut feelings, mistaking reflex revulsion for ethical insight.
First of all, this is far from the only logical way to make sense of it; Baggini has just quoted someone noting that one reason for being reluctant to jump on the bandwagon is that it might feed into the sentiment that meat-eating is really the only normal way to go and that the whole vegetarian and vegan approach was just a quixotic attempt to be abnormal all along. Further, Baggini has also just pointed out someone whose response was simply, Why would one go through so much trouble for something so unnecessary?
Second, Baggini is conflating reasons for becoming vegetarian with reasons for remaining vegetarian; people who become vegetarians may genuinely become vegetarians for purely animal suffering reasons and then become convinced that there are other ethical reasons for being vegetarian, such that even if you could remove animal suffering, they would have principled reasons for not changing back. Nothing Baggini has said shows otherwise.
Third, Baggini's evidence here is a bit of mish-mash -- we have several quotations from vegetarian organizations which give reasons for their reluctance, then we have a reference to a poll that is not about backing lab-grown meat but about whether they, personally, would eat it. There is no particular reason to conflate the two; indeed, Baggini goes on to quote someone who 'backs' it but says they wouldn't eat it themselves.
Fourth, setting aside the fact that nobody has 'unmediated' gut feelings, I very much doubt anyone confuses their reflex revulsions for ethical insights; rather they, fully recognize that they are reflex revulsions, and reasonably refuse to jump on the bandwagon supporting things that revolt them unless they are actually given good positive reasons to do so, which 'Now with less animal suffering' is not. (To get a sense of this, think of cloning human muscle tissue in the lab. No humans were hurt in the making of this human flesh! So now you personally are ready to start eating human meat, aren't you? And you'll enthusiastically back attempts to start selling it in the supermarket?) You can't throw all of human motivation out of ethics; nor can any reasonable ethical position depend on demanding, at every drop of a hat, that people back things that revolt them.
Fifth, Baggini goes on to say, " But there is a huge difference between building your position on a firm evidence base and building an evidence base to support your position." This is a common view, but it's quite clearly wrong. In practice there actually isn't all that much of a difference between building your position on a firm evidence base and building an evidence base to support your position. There are some biases that can be dealt with more easily the former way than the latter, yes; but so little does it matter whether a position is reasoned out or rationalized that it rarely makes a difference to the overall state of the argument. The reason for this is that motivated reasoning does not result in fundamentally different kinds of arguments from those built by open-minded reasoning, and in public discussion they are subjected to exactly the same standards. Further, dismissing the arguments because they are rationalized is what is often called the 'genetic fallacy', and, indeed, a paradigmatic example of why the 'genetic fallacy' got the 'fallacy' title. Psychologically, even if an argument is developed out of rationalization, that does not mean that it is not a good argument, anyway -- particularly if it withstands the test of objections and criticisms, one might well be foolish to give it up, no matter how you got it. It's absurd to treat a methodological principle about the best way to approach a subject in inquiry as if it were some overarching matter of ethical and rational integrity. Even if Baggini is right that vegetarians are merely rationalizing -- and he has not done a single thing of significance to show it -- what does it change? Nothing whatsoever. It wouldn't magically take the arguments they do give off the table, and it wouldn't change the standards of argument that have already been operative all the way through. When rationalizations are bad, it's never because they are rationalizations but because they are absurdly biased, or sloppy, or have gaping holes you can drive a truck through; all of which they could very well have if they had not been rationalizations at all. Rationalization is more likely to produce weak or bad arguments, yes; but it is neither a necessary nor a sufficient condition of them, and since rationalizations produce products of the same general kind, even if not consistently the same quality, as any other kind of reasoning, pretty much everything else stays exactly the same whether an argument is a rationalization or not.