And it has been proven, time and again, that a vegetarian diet is healthier for the body, the mind, the environment, and society.
I'll give you the environment, hands down. Not sure what you mean by society, so I'll ignore that one. The body? Yes, IF the person knows how to form a nutritous vegetarian diet (many people new to vegetarianism don't). The mind? I don't know about that one. I think what's healthiest for the mind diet wise is probably what makes you happiest, and may well be in contrast to what's best for the body and environment. For most people being an ominvore is probably what's best for their mental state; those who aren't I imagine are either very environmentally conscious, or have a high regard for animal rights and thus probably derive more satisfaction from the "good" their diet does than the enjoyment that delicous bacon would provide.
At any rate, I'd agree with you that people shouldn't attack vegetarian or vegan lifestyles. I also think though, that those who have chosen those diets should realize that a holier than thou attitude won't make inroads with meat eaters, even if they have a point.