It seems to me that personal convictions (ie religious) are what drive the VAST majority of mainly females to a vego diet. From my perspective as a bodybuilder this is positively the WORST reason to adopt this diet. It explains why vegetarians have a bad reputation as being skinny, sick, weakly, pale etc.....all pretty much true on average.
On the other hand, if you are interested in good health and building a better body to last for a lifetime, then a well thought out-PLANNED-vego diet is very beneficial for men and women in my experience. You don't see many good quality vegetarian bodybuilders though because 1)The religious reasons that most vegeos have for becoming vegetarian are at odds with the sport of bodybuilding and 2) most vegos are frighteningly ignorant of proper nutrition. They tend to read (and believe) popular books and websites devoted to their cause (can you say 'agenda to sell') rather than taking an unbiased nutritional/scientific approach to designing their diets. The few vegetarian bodybuilders I have worked with (including myself) all have an uncanny ability to gain more muscle and to get leaner for competitions than their omnivorous counterparts. But that's all anecdotal, and may just be because they follow meticulously drawn out, very high quality diets.
The non emotional benefits I have found are that it's hard to overeat to the point of obesity; it's easier to get lean and retain LBM; it can be cheaper (especially during growth phases); you do not put yourself at as high a risk of diseases such as BSE, E.coli, salmonella etc..., food doesn't smell so bad when it goes off in your fridge; your poos don't stink to high-heaven (instead they merely stink to mid-heaven); heart disease and diabetes are pretty rare amongst even ill-educated vegetarians; large, moist; regular bowel movements......
The disadvantages include social difficulties when eating out or entertaining people; a majority of people who think (even if they don't say it to your face) that you're a nut; saying no to good smelling foods like chicken soup, bacon or a supreme pizza, which often leads to wasting money on fake meat products that are even worse for you than they meat they are replacing, spending large amounts of time chopping veggies, boiling beans, chickpeas, brown rice and other slow cooking grains; flatulence; bloated stomach........
That's just the short list.