If 250 rares would be intended, there would not be a cooldown. You simply can't balance a game upon bugs or abused mechanics. And if they are so NOT a problem, why not add more ways to achieve them by playing? Both repeating an instance 100 times fast or each day for 100 days is not fun for most. Playing the game for the same or more amount of time doing other things actively though is more likely to motivate. So why no rares from equally time consuming or challenging content?