Dolphins are smarter than humans, one of the only species. If they're allowed to roam free, it won't take long for dolphins to figure out how to make underwater technology. Once they do, they'll systematically take over the earth and wipe out humanity. It's imperative that they're kept in a location where they won't be able to wipe us all out.
Dolphins have been used as an inexpensive pork substitute for centuries. A lack of captive-raised dolphin meat would result in a moderate rise in pork prices, which would be a disaster for the world's impoverished who depend on what little SPAM they can afford for life giving protein. However, I can't think of any reason why they should be kept for entertainment purposes. That would just be cruel.
Dolphins are indeed smart, intelligent, and amazing creatures that swim through the oceans and seas gracefully and peacefully. They are sentient just as we are and feel pain, sadness, and happiness just like us. So, why do we imprison them in tanks just for our selfish desires? I don't think dolphins would jump out of the ocean and gun down humans in cities if let free. Instead, they would be able to live happy, peaceful lives in the wild. After all, how would you feel if aliens enslaved you, threw you in cages where other aliens would stare at you as other aliens beat you and forced you to perform for their "entertainment."
No, because dolphins are very intelligent animals, and they have a sense of self. To keep them in small tanks is like placing a human in a room of about 20-30 square feet for it's life! It is unreal and cruel to do this to these amazing animals. FREE THE DOLPHINS!!!
Dolphins in captivity is similar to orcas in captivity. They are too sentient to be imprisoned in theme parks and exploited for entertainment purposes. Put yourself in their shoes: How would you feel if you were taken away from your family and placed in a tank at a theme park where you were forced to perform?