+15 hp healed for adjacent units is HUGE. It's like having 3 medics. It's useful throughout all ages (unlike every other UU), and it lets you tank city + garrison fire if you have a tech advantage. It dramatically slows down the "wait 5+ turns to heal up damage" snorefest when your units get heavily damaged. Combined with march, my units can attack, retreat, and be healed up next turn. Without the khan's healing bonus, my mounted units would have to retreat for 2-3 turns after performing a melee attack.