For educational purposes, yes, i think it is important to have aquariums with sharks in them, but of course with in reason. Some sharks do very well in captivity, others, like the Great White, do not. I always enjoyed going to aquariums when i was a kid, it is what inspired me to want to learn more about sharks and our ocean and to become a SCUBA diver. Sharks are so awesome, and have such an awe factor, that an aquarium without a shark usually doesn't do as well for attracting visitors to learn about the ocean wildlife. (I personally would rather go somewhere I can see a shark over seeing a dolphin). I also think that if the shark(s) in an aquarium were found injured, and after rehabilitated could not return to the wild, then that is also an acceptable case for keeping a shark in an aquarium (or any other marine life). I am not fond of "bred in captivity" or "hunted for in the wild"...
For example, sea turtles involved in boating accidents that lost some of their fins, or became partially paralyzed in and would not survive in the wild. but at the aquarium they thrive! So in those cases, it not only helps the animal, but it also continues the out reach to the public, bringing awareness to our great oceans, and to why following wake zone rules are important etc.
Most aquariums do a good job of keeping their marine life happy and healthy, and many of them may live beyond their expected life span because of this. Again, within in reason, i think Aquariums are good for public out reach, and education of our oceans and its creatures, and when possible, the animals should be rescue animals that needed human help or they would not have survived.