Amino acids can be very beneficial to ones health. The addition of specific amino acids to the diet have been proven to help clear up specific health ailments in certain individuals (ranging from migraines, depression, fatigue, insomnia, and other issues). If you are exercising, especially weight bearing exercises, amino acids are very beneficial for recovery, tissue regeneration, & muscle growth.
If you understand the art of food combining you can get most amino acids from your diet. The only vegetarian food that singly contains all amino acids to make up a complete protien is hemp seeds. Soybeans come close.
I think it can be a wise health choice to supplement w/ a good liquid amino acid formula if you are a vegetarian who is not especially consistant about proper food combining.