No reason for that setting is to avoid persistent notification.
Remember, it's "putting people first and reinvented around you"
So if you had 5 SMS. You tap on tile, only reply or read one, the notifications clear away. Why? Because what was important to you according to the phone, was brought to the live tile for you. You chose to only reply to the most important SMS. Hence, the other 4 SMS that aren't important, won't stay on your live tile, bothering you. It will go away. So you have fresh place for new content.