Oh sure, eating two quarter pounders with cheese and large fries every day will certainly cause you to gain a lot of weight while gaining few nutrients. However, simply cutting the meat out of those quarter pounders will not help the person much--and that is what I mean.
I think that many believe that if they do nothing else other than not eating meat that they will be healthy and lose weight -- which is misguided. You can be perfectly healthy and feel great without having to give up meat--the best source of protein.
If a person transitions from a fast food diet to a vegetarian diet and loses weight--its because they've also made other changes to improve their diet and health. Cutting meat is not necessary for the vast majority of people.