Control Voice wrote:
. . .Plus it would have a built in tension as history shows he lived out his life in his own era, so Kirk and Co. know they will return him to his proper place in time. . .
Which means he would have been too important to send on landing parties, and be stuck on a ship he didn't want to be on in the first place, and had already tried to escape from. It's not that the crew knows they'll return him to his place in time, it's that they have to keep him alive long enough to make the attempt that they're not even sure will work.