For example, the Animal Activist's Handbook calls health-based arguments "problematic" and urges readers to focus on ethics-based approaches. No less an authority than Mahatma Gandhi said in his book The Moral Basis of Vegetarianism:
I notice also that it is those persons who become vegetarian because they are suffering from some disease or other - that is, from the purely health point of view - it is those persons who largely fall back. I discovered that for remaining staunch to vegetarianism a man requires a moral basis.Like a lot of marketing advice, these theories are usually justified by an appeal to intuition, and like most such appeals I suspect that they aren't well supported by the facts.
A review of US meat consumption found that health information (as measured by the number of articles published in medical journals about the bad effects of cholesterol) had a stronger effect on demand than even price changes. A similar review of Canadian meat consumption found that government recommendations to eat less meat appear to have a significant impact. Concerns about cholesterol have sent the demand for butter and eggs plummeting. As Oprah fans know, information about the unhealthfulness of beef causes a huge drop in beef consumption - without increasing the consumption of pigs or chickens.
In a survey by the Vegetarian Journal, 82% of readers stated that they became vegetarian for health reasons, and among adolescents a vegetarian diet seems to be linked with a desire for weight control. This is confirmed by the Vegetarian Times' survey, which found that the majority of self-described vegetarians do it for health reasons. In a psychological survey of the origins of vegetarianism, the authors found that slightly less than half of vegetarians originally quit eating meat for health reasons. Vegetarians of all stripes are significantly more likely to be concerned about health aspects of their food.
And we shouldn't think that someone who becomes veg*n for health reasons will be less committed. An attempt to understand the process of becoming vegetarian found that slightly more than half the subjects were vegetarian for ethical reasons, but "health vegetarians became increasingly aware of animal welfare issues and this reaffirmed the transition." Indeed, the initial ethical/health distinction seems to fade over time as ethical vegetarians become more interested in health, and vice versa.
Unspeakably more depends on what things are called, than on what they are. - Friedrich Nietzsche
It's critically important to consider here too the benefit gained from advocacy that is not "vegan advocacy." You probably have heard of pink slime, a filler used in ground beef. The public outcry sent beef prices plummeting, causing at least one producer to declare bankruptcy. Several lawsuits regarding E. Coli-infected beef caused Topp's Meat Company to file for Chapter 11 a few years ago. The Hallmark/Westland Meat Packing Company went bankrupt after an investigation by the Humane Society of the US caused the largest beef recall in history - not because of animal cruelty violations (which were horrendous), but because of health concerns.
Bruce Schneier has said that no one should be concerned by what's on the news - if it's newsworthy, it's by definition unusual, hence it almost certainly won't affect you. This is a fact which a lot of advocates seem to forget. Pink slime is probably no worse than any other type of meat, yet some combination of branding, luck and timing caused tremendous economic damage to the beef industry. Similarly, your chance of dying from E. Coli even during an "outbreak" compares favorably with that of being struck by lightning, yet we find massively expensive recalls happening on an almost weekly basis.
So we have to be extremely careful when evaluating things like the evidence that vegan diets help with long-term weight loss. They stack up pretty well when compared to the competition, but the fact that they aren't overwhelmingly better than anything else doesn't necessarily mean that the health argument fails veganism.
Maybe health benefits aren't the best way to present veganism. Certainly there is a subgroup of people that is more responsive to ethical arguments than health ones, and we have to be careful about change which moves people from one type of animal consumption to another (although the evidence seems to indicate that this is less of a problem than one might think). But I hope I've convinced you that this is not something which can be decided by navel-gazing - it needs to be decided empirically, by doing surveys, handing out pamphlets and measuring what works.
If you are interested in learning more about health-based arguments for veganism, PCRM is a good place to start.