Oh, I don't know about this. How “sentient” would an organism have to be before it would be cruel or dangerous? As to bringing an extinct species back, lots of folks wouldn't mind if the Dodo came back, or the Carrier Pigeon, or the Tasmanian Tiger. I mean, if they're back, then they aren't extinct. The Geico Neanderthals get their feelings hurt all the time, but I think they'll get over it eventually.
I like the idea of bringing the Mammoths back. They would keep the bear and mountain lion population down out here where I live. Wouldn't help my landscaping much though. :-(
Neanderthals were sentient. How would you like to be the only member of your species brought back just to be prodded and studied. What would his/her immune system be like in relation to today’s diseases? Where would they fit in? Besides, the 0bama EPA would never allow cloning mammoths because of their carbon footprints.