Vegetarianism or Happy Pigs

(Image from bbc.co.uk - website below)

I read this interesting article about a man who was a vegetarian but now farms his own pigs, which he eats.

Tom Welch, the farmer, said that his reasons for being a farmer were ethical farming ones - he believes it is wrong to keep animals in poor conditions, and that there is no clear link between the meat we buy in the supermarkets and the animal it came from.

One comment by a reader at the end struck me:
"Tom Welch has clearly never been a true vegetarian. His reasons for being so were out of concern for animal welfare during farming. But the true reason for being vegetarian is that you believe it is morally and logically wrong to kill other creatures in order to eat their flesh when it is possible to live off grains, pulses, fruit, vegetables, and nuts."
- Neil, County Antrim

Is there are "true reason" for being a vegetarian? What do you think? Should people be vegetarians? If so, why? Are some reasons better than others?