Note: I am, of course, speaking largely from personal opinion after looking into this subject and after seeing many of the results of various types of training and “domesication” scenarios. I am not saying views that disagree with mine are wrong entirely.
I don’t believe that wild animals should be kept and maintained for unnatural purposes at all. If you subscribe to the human-animal bond, you concede to the fact that the mutual bond you share with a domestic animal is the result of thousands of generations of co-evolution and development between our species.
Dogs, cats, and other domesticated animals have come to fulfill needs for each other. Part of a healthy “puppyhood” for a domestic dog, for example, involves just as much human exposure as it does other dogs in the 6-12 week mark. Likewise, it is just as important for a puppy to learn how to behave socially with humans as it is other dogs — dogs communicate in complex heirarchies and a series of submissive and aggressive behaviors, but with humans it’s just as important. Dogs’ brains develop healthier and with more capacity for intelligence and learning when they are worked with consistently and made to understand human commands and interactions.
To a reasonable degree, you could healthfully and understandably train many domesticated animals to perform and it not be an aggressive removal from their natural environment — one which already incorporates the cohabitation of us strange bipedal man-beasts.
With wild animals it’s entirely different. I do not like any version of training them because it will inherently be against any healthy and natural stimuli they have.