There is an overwhelming abundance of information, some based in statistical evidence, some based in 'diet guru' theory, that would tout the benefits of almost any diet imaginable. I recently saw one 'healthy fast food diet' that showed people how to eat at Mickey Ds everyday without becoming a fatass. Super gross.
Regardless, one would be hard pressed to come up with a good reason not to eat a diet full of fresh, local foods and avoid processed, sodium-and-sugar filled crap. I've been eating a vegetarian diet with very few animal products for about a decade and can't say enough great things about it. You get to enjoy so many more fantastically delish things, and fresh, vegan ingredients are surprisingly inexpensive. Plus (and we all know that this is the really important part), I live in Texas and I know about 147 fat people. Not one of them is vegan.
Factory farming, environmental damage, and the need to find sustainable food sources for an increasingly hungry world population should be enough to make anyone with a conscience pay attention to what they put in their mouths. For some more reasons to go veg, check out:
http://www.geocities.com/~newliberty/robbins.htm