Very good comment!
Yes, it's always the question if the positive or negative aspect of keeping large animals prevails.
The positive aspect can be that more people (especially children) learn to know and value the creatures and later try to help protecting them. Also some rare animals are cultured in zoological gardens.
And yes, true, the negative aspect is that the animals would have a much better life in their natural environments.
Yes, good points. There are more arguments that proponents of keeping animals in captivity often use, such as the fact that the animals are protected from predators and dangerous situations that they would have to face in their natural environment etc. On the other, the opponents argue that all creatures are always better off when left living their natural life and just following their natural instincts. I would say it depends on the particular animal (I believe some species can live happy lives in captivity when given proper care and living conditions) and the particular keeper and his or her attitude.
I remember from one of our previous conversations (I think it was under my post about scorpions) that you are a dedicated nature lover, just like me, so I wondered what your take on this thing is :)