I would assume that all the Connies got different mission types from deep space research to petrol duty to peaceful diplomatic mission. I assume also that the USS Enterpise was the only ship to seek out new life and complete its mission !!!! E.G. The USS Intrepid was lost with all crew investigating the space creature. I always thought that they must have been captains with the more experience to handle situations and kirk couldn't be the only legendary captain in starfleet???