I think it would be beneficial to overall replayability and fun factor to make sure rare spawn monsters are present in the game. Make them hard to get to. Make them over powered. Make their drops extra special and extra rare. like weapons, armor or crafting ingrediants. they can be on all sorts of timers like 24hr spawns, once a week or month even. random spawns and so on. As far as global bosses go we need monsters that are so huge that if there isn't at least 50 people willing to fight it theres no chance in hell of beating it. Many large MMO's implement this in their games and it makes it fun to get together with a bunch of friends and go "hunting"... or hunted.