Your body needs protein. That's something you can't get eating all plants. Besides, the chemicals they have to use to get plants to taste like real meat can't be good for you. They never mention the chemicals they use.

A coworker tried it for about a year. His health slowly declined, low energy, sick more often. He was eating meat bin short order.

Vegans avoid all animal products, including meat, dairy, eggs, and honey, opting for a completely plant-based diet.

Vegetarians do not eat animal flesh (meat, poultry, or fish) but may consume dairy products, eggs, and honey.

Vegans are a bit more hard core i guess.
Keiths right about the gas, it's friggin terrible. If a guy could harness it somehow it may be beneficial as an alternate source for acetylene.