Mid day groomings knock down piles. Thats all. When they groom a trail at 11pm it has time to sit groomed, and the snow 'sets up'. It gets firmer the longer it goes without being groomed. If you groom a trail and then let people ski on it right after, its like flattening sugar...when it comes down to it, as soon as a few people ski over it, it will be piled up again.
Then again, some days the piles are so bad, its not a bad idea...if you need stuff like that to help you ski
I like night skiing when everything is scrapped down. But I don't like it when there are those little rollers (unless I'm just screwing around with friends, then it doesn't really matter)