We definitely keep SOME whales in captivity in order to preserve the species. Their captivity helps sensitize people to their inherent majestic beauty. They should nonetheless be kept in more than ideal conditions, including adequate space, care, and quality food. And we should not over do it. Just a few in captivity will do.
We should no longer keep killer whales in captivity. Recent events have shown that this practice is detrimental to their health and that the public is not always receptive to the idea of using these animals for show. These animals are also so large that keeping them in captivity is very cruel as they do not have the space to explore as they would in the wild, nor do they continue to act as they would in the wild.
How many people does Tillikum have to kill before we get the message. Captivity has turned him truly into a "killer whale". The pools are not big enough and you're sticking random whales together who may not get along. This leads (just like with people) to anti-social behavior. Its just not worth it.
Studies show that Orca whales living in captivity have higher levels of stress that cause psychological and physical deterioration, and ultimately shorter lives. When you take an intelligent wild creature who's evolved to live a social life with other whales in the wide-open ocean, and then stick it in a tank to die alone, it's not wonder he might become aggressive or depressed. It's cruel and exploitative to keep these whales in captivity purely for our enjoyment.